FABRIC.md: resolve §17.3 -- words onto the Stadium, hosted/kernel split

Elevates §17.3 from LEANING to DECIDED, settled on paper before item
4.1's code per Captain Bob's request. The core claim (replace
round-robin eviction with Stadium density ranking) already argued for
itself in this section; what was missing was the hosted/kernel split
§25.5's "never two live heat mechanisms at once" implied but never
resolved.

Grounded in code, not policy: dictionary_management.c calls
hotwords_cache_lookup()/evict_*() unconditionally in the word-lookup
path (not gated by ENABLE_HOTWORDS_CACHE at the call sites), and that
file is vendored shared source required to work in both hosted and
kernel builds. The Stadium is kernel-only by construction of
everything built through item 3.7. ENABLE_HOTWORDS_CACHE already
defaults off in both Makefiles today (Kconfig.physics, verified
against both Makefiles directly).

Resolution: kernel builds retire the old cache's effect once item 4.1
lands (Stadium takes over, old call sites bypassed under
__STARKERNEL__ regardless of the Kconfig setting); hosted builds are
unchanged, no Stadium is built for them. Item 4.1 decides the exact
bypass mechanism, not invented here.

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

FABRIC.md

@@ -823,8 +823,55 @@ Consequences if this holds:
 - Pin stops being a general-purpose escape hatch and goes back to meaning what §3 says:
   invariance, for the few things that genuinely must not vary.
 
-**LEANING.** The mechanism is already built and the fit is clean, but this reframes a
-direction stated differently earlier the same day, and it deserves longer than a paragraph.
+~~**LEANING.**~~ **DECIDED 2026-08-04**, on paper, before item 4.1's code — settled after
+starting that item surfaced this section as its unresolved prerequisite.
+
+**The core claim stands, for the reason already given above.** The existing cache is
+self-contradictory as built: heat gates admission, nothing governs departure, and the
+consequence is that the hottest word in the cache can be evicted purely because its slot
+came up in the round-robin rotation. Moving words onto the Stadium repairs a real defect,
+not a relabeling exercise, and `stats.evictions`/`stats.promotions`/`stats.cache_hits` make
+it directly measurable before and after, exactly as this section already argued.
+
+**What was actually missing was the hosted/kernel split** — §25.5's own header warns
+"never two live heat mechanisms at once" (§11), but nothing said which way that resolves,
+and the two subsystems involved are not symmetric:
+
+- `src/physics_hotwords_cache.c` and `src/dictionary_management.c` are vendored, shared
+  source. `dictionary_management.c` calls `hotwords_cache_lookup()` /
+  `hotwords_cache_evict_*()` directly in the word-lookup path, **not gated by
+  `#ifdef ENABLE_HOTWORDS_CACHE`** at the call sites — the toggle only controls the cache's
+  own internal behaviour, not whether these call sites exist. This file must keep compiling
+  and working correctly in both the hosted and kernel builds (CLAUDE.md is explicit).
+- The Stadium (`stadium.h`/`stadium.c`) is, and by construction of everything built through
+  item 3.7 can only be, kernel-only — every declaration in it is `#ifdef __STARKERNEL__`.
+  Nothing in this document has ever proposed a hosted Stadium, and building one is not
+  something item 4.1 needs to do.
+- `ENABLE_HOTWORDS_CACHE` already defaults to **off** in both the hosted `Makefile` and
+  `Makefile.starkernel` today (`Kconfig.physics`, confirmed against both Makefiles directly
+  — a stale comment beside the Makefile default claims the opposite, but the actual default
+  is `n`/`0` in both). So "two live mechanisms" is not a live conflict in the default build
+  today; it only becomes one once item 4.1's kernel-side migration and the old cache are
+  both actually exercised at once.
+
+**Resolution: kernel and hosted diverge, and that is the correct shape, not a compromise.**
+
+- **Kernel builds:** once item 4.1 lands, the old cache's *effect* is retired under
+  `__STARKERNEL__` — word patrons migrate onto the Stadium, and
+  `hotwords_cache_lookup()`/`hotwords_cache_evict_*()`'s call sites in
+  `dictionary_management.c` are bypassed for the kernel build regardless of the
+  `ENABLE_HOTWORDS_CACHE` setting. The shared source can stay compiled as-is (untouched, for
+  hosted's sake) while being functionally inert on the kernel side. Item 4.1 decides the
+  exact mechanism (a build-time gate, a runtime check, or something else) — not invented
+  here.
+- **Hosted builds: unchanged.** No Stadium exists there, none is being built for it, and the
+  existing mechanism — including its current off-by-default setting — stays exactly as it
+  is. This is what keeps CLAUDE.md's dual-target compileability requirement satisfied
+  without inventing a second Stadium implementation nobody asked for.
+
+Consequences from above still hold: words need no pin exception (reap event is cooling off
+the floor), §16.3's dictionary-parity concern narrows to the hot set alone, and pin goes
+back to meaning invariance rather than a general-purpose escape hatch.
 
 ### 17.4 Open