FABRIC.md: close §17.7 -- reservoir-based O(1) word heat transfer, DECIDED

Resolves the last open question: vm_physics_touch()'s O(n) proportional
fan-out (justified there only by rare fleet touches) isn't viable at word
dispatch frequency. Instead each VM's inner Stadium gets one reservoir
scalar; touch/cool/evict are all O(1) two-party transfers against it,
mirroring Hera's structural role at the fleet level rather than
peer-to-peer redistribution. Admission is the starter grant itself
(Option B): execution_heat stays fully inert in kernel builds, matching
the one-governor rule already committed, at the cost of running
stadium_admit()'s existing O(N) scan on every cold-word dispatch until
item 3.5's free list lands -- accepted debt, not a new gap. Corrects
abb1d92's invariant wording: residents sum to less than Q48_ONE, with
the reservoir holding the remainder.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

FABRIC.md

@@ -1078,7 +1078,7 @@ that matters. The total stays fixed; only the partition shifts.
 Decide how big the Stadium is at runtime. Do not decide what an entry is, or how entries
 are addressed, at runtime.
 
-### 17.7 Word-level heat conservation — LEANING, one question open, blocks item 4.1
+### 17.7 Word-level heat conservation — DECIDED 2026-08-05, blocks item 4.1 until implemented
 
 Item 4.1 (hot words onto the Stadium, §25.5) needs word patrons to carry a Stadium `heat`
 share. The obvious move — `q48_from_u64(execution_heat)`, a pure representation change
@@ -1108,15 +1108,51 @@ resolves four of the five questions this section left open:
    today; the counter keeps incrementing and decaying exactly as today. Stadium `heat` is not
    part of `dict_hash` and item 4.1 does not need to add it there. No baseline discontinuity,
    no "before vs. after" comparison problem — there is nothing to reconcile.
-3. **What transfers, from whom, on every dispatch — and its cost.** **Still open.** This is
-   real work, not dissolved by the reframing above: on a kernel-build word dispatch, some
-   Q48.16 amount has to move toward the touched word's cell from the rest of the VM's
-   resident pool — the same shape as `vm_physics_transfer()`'s clamped subtract/add at the
-   fleet level, or the proportional redistribution at `capsule_vm_physics.c:334`, but nobody
-   has picked the exact rule or measured its cost on this path yet. This sits in both the
-   hosted and kernel dispatch primitive today (`physics_execution_heat_increment()`), but
-   under the resolution below the *new* transfer is kernel-only, alongside — not replacing —
-   that existing increment. Still blocks item 4.1's code.
+3. ~~**What transfers, from whom, on every dispatch — and its cost.**~~ **RESOLVED
+   2026-08-05 — a reservoir, not a fan-out, keeping the transfer O(1).**
+   `vm_physics_touch()`'s proportional pull across every other live VM (`capsule_vm_physics.c
+   :281-355`) is O(n) over the fleet and is explicitly justified there only because fleet
+   touches are rare — the file's own comment (`:47-49`) contrasts "dozens to low hundreds" of
+   fleet touches against word executions "in the millions." Copying that shape for words is
+   not viable.
+
+   Instead, each VM's inner Stadium gets one additional scalar — the **reservoir** — holding
+   whatever heat is not currently claimed by a resident patron. All word-heat transfers are
+   two-party, against the reservoir, mirroring Hera's structural role at the fleet level (a
+   single fixed point that absorbs and donates) rather than the fleet's peer-to-peer fan-out:
+   - **Touch** (dispatch of an already-resident word): pull a fixed Q48.16 quantum from the
+     reservoir into the word's cell, clamped at what the reservoir holds. O(1).
+   - **Cooling** (Loop #3's decay shape, redirected): return heat from the cell to the
+     reservoir instead of letting it vanish. O(1) per word, same as today's independent decay.
+   - **Eviction:** the cell's *remaining* heat must flow back to the reservoir before the cell
+     returns to the free list (item 3.7) — otherwise conservation breaks on every reap.
+   - **Quantum size:** a Kconfig constant, not an inferred rate. The fleet needed a
+     statistical estimator (`VMFleetWindow`, `vm_physics_tick()`) because its touches are rare
+     and irregular; words already have a simpler precedent — `execution_heat`'s existing
+     per-dispatch increment is a flat `+1`, not tick-scaled. Mirroring that shape avoids a
+     second estimator. Actual tuning is DoE work (item 5.1), not decided here.
+
+   **Admission is the starter grant, by explicit choice (2026-08-05) — `execution_heat`
+   plays no role.** A non-resident word has no cell, so its density is 0 and it can never win
+   item 3.5's "denser than the least-dense resident" comparison on its own. Two shapes were
+   weighed: (A) gate admission attempts on `execution_heat`'s existing threshold crossing —
+   free, since the increment already happens, but makes `execution_heat` the promotion
+   governor in kernel builds, directly against the "one governor per build" rule below; or
+   (B) every dispatch of a non-resident word requests a fixed starter quantum from the
+   reservoir and is admitted iff that quantum's density beats the current least-dense
+   resident — `execution_heat` stays fully inert, matching the rule as already committed.
+   **Chosen: (B).**
+
+   **Accepted cost, not a new gap:** `stadium_admit()` (item 3.5, `DONE`) already documents
+   its own two full-array scans as O(N), deferred until the free list lands (`FABRIC.md
+   :2971-2974`, "not worth a workaround for code with no caller yet"). Option B calls that
+   O(N) path on every dispatch of every non-resident word — the same "check on every touch"
+   shape the old hot-words cache already used (`hotwords_cache_lookup()` in the word-lookup
+   path, §17.3), but each check costs O(N) here instead of the old cache's O(1)
+   threshold-plus-round-robin-write. This is deliberately paid for now, in exchange for
+   keeping `execution_heat` inert, and is expected to resolve the same way item 3.5's own
+   scans do — when the free list (item 3.7's per-VM free-list heads) replaces the full-array
+   scan. Not a new item; it collapses into item 3.5's already-recorded debt.
 4. ~~**What happens to `dict_hash` and parity comparisons that predate this change.**~~
    **RESOLVED by #2 above** — nothing predates a change that isn't being made to the hashed
    field.
@@ -1124,14 +1160,22 @@ resolves four of the five questions this section left open:
    The Stadium engine (§18) already owns a conserved heat wire per cell; item 4.1 populates
    that existing wire for the word patron kind. It is not a new loop and needs no name.
 
-**What sums to what, and admission semantics — settled by code already written, not
-re-derived here:** per-VM Stadium, summing to 1.0 across every patron resident in that VM's
-quota (words, blocks, ACLs, messages together, not a word-only sub-pool) — matching §21.4's
-"K conserved here, independently" and the `stadium.h:64` field comment. No reset on
-admit/evict: the sum stays invariant across *any* call per §19.1's own conservation check, so
-admission is itself a transfer, not a reset — mirroring `capsule_vm_physics.c`'s VM-birth
-pattern, where a new patron starts at 0 (or a starter share) and is topped up by transfer,
-never by manufactured heat.
+**What sums to what, and admission semantics — settled by code already written, plus the
+reservoir above:** per-VM Stadium, one pool per VM (words, blocks, ACLs, messages together,
+not a word-only sub-pool) — matching §21.4's "K conserved here, independently" and the
+`stadium.h:64` field comment. **Correction to this section's 2026-08-05 earlier wording:**
+the invariant is not "residents sum to `Q48_ONE`" — the reservoir holds whatever residents
+haven't claimed, so the correct invariant is
+
+> Σ(resident patron heat) + reservoir == `Q48_ONE`
+
+checked the same way `vm_physics_conserved()` checks the fleet sum, epsilon-bounded. No reset
+on admit/evict: the total stays invariant across *any* call, so admission and eviction are
+transfers against the reservoir, never a reset — mirroring `capsule_vm_physics.c`'s VM-birth
+pattern (a new patron starts at 0, topped up by transfer) with the reservoir playing Hera's
+role: at VM-Stadium-quota-grant time, before any word patron is resident, the reservoir holds
+the VM's entire share, exactly as Hera holds the fleet's entire `Q48_ONE` before any other VM
+is born.
 
 **One governor per build — states explicitly what closes the §11/§25.5 "never two live heat
 mechanisms" gap:**
@@ -1146,20 +1190,24 @@ mechanisms" gap:**
 This is the same per-build split §17.3 already ruled for the cache itself; word-level heat
 conservation follows it rather than inventing a second shape.
 
-**Open, to settle before item 4.1 writes any transfer code:**
+  **Open, deferred honestly rather than blocking:**
 
-- The exact per-dispatch transfer rule (pull from a proportional share of current residents?
-  from a designated "unclaimed" pool representing non-resident dictionary words? something
-  else) and its measured cost on the dispatch path, kernel build only.
+- The quantum size (Kconfig constant, §17.7 bullet 3 above) has no value yet — tuning is DoE
+  work (item 5.1), not invented here, same treatment as `STADIUM_MEMORY_PERCENT` (item 3.2).
 - Whether `rolling_window_seed_hotwords_cache()`'s POST warm-start
   (`rolling_window_of_truth.c:786`) needs a Stadium-side counterpart to seed word patrons'
-  initial `heat` distribution — noted here as the natural seeding site, not designed.
+  initial `heat` distribution from the reservoir — noted here as the natural seeding site,
+  not designed. Item 4.1 may ship without it; POST warm-start of the *old* cache is unaffected
+  either way since it writes `execution_heat`, which stays untouched.
+- `stadium_admit()`'s O(N) scans (item 3.5's own recorded debt) are accepted cost for word
+  admission under Option B, not re-litigated here. They resolve when the free list (item 3.7)
+  supersedes the full-array scan — tracked at item 3.5, not a new item.
 
-**Nothing in this section authorizes touching the per-dispatch transfer rule, or wiring
-`stadium_admit`/`stadium_dispatch` for words, before the open bullet above reads DECIDED.
-`execution_heat`'s current increment/decay behaviour and `dict_hash` are explicitly out of
-scope for item 4.1 — this section's earlier ban on touching them no longer applies, because
-nothing here proposes touching them.**
+**This section now authorizes item 4.1 to wire word patrons onto the Stadium using: the
+reservoir-based O(1) touch/cool transfer, Option B's starter-grant admission (no
+`execution_heat` involvement), and the corrected invariant above. `execution_heat`'s current
+increment/decay behaviour and `dict_hash` remain explicitly out of scope — nothing in item
+4.1 touches either.**
 
 ---