Stadium: real block-patron admission + MIGRATE dispatch (FABRIC-3.md §B)

stadium_admit()'s mass==1 refusal looked like a hard blocker for 1024-byte
blocks, but stadium_word_dispatch()'s real candidate construction proves
Stadium cells carry pure identity/heat/bookkeeping, never the resident's
actual content -- a block patron follows the same shape (identity=LBN,
payload unused), so this was real, scoped work, not a case for stubbing.

New stadium_blocks.h/.c mirror stadium_words.c's admission/cooling shape,
keyed by (quota_slot, lbn) in a fixed-capacity open-addressing hash table
(tombstone deletion) instead of a dense array, since LBN space isn't
densely bounded like word_id. Wired into block_word_block()/buffer()/
update() (block_words.c), __STARKERNEL__-guarded. stadium_dispatch()'s
MIGRATE case now calls blk_flush(lbn) for real instead of printing
"(stub)". Three new Kconfig constants (STADIUM_BLOCK_HEAT_QUANTUM/
STADIUM_BLOCK_COOL_RATE_Q48/STADIUM_BLOCK_TRACK_CAP_MULT) mirror the
word-patron ones, same three-layer wiring.

VM-COOL/DELIVER/EXPIRE stay explicit punch-list items -- VM-COOL
deferred pending the still-iterating Tripod/Zuse/messaging vision,
DELIVER/EXPIRE are their own future subsystem integrations per
FABRIC.md's own "open, not resolved" notes.

Verified clean compile (zero warnings) and clean boot to REPL with
conservation intact (resident_sum + reservoir == Q48_ONE) on all three
architectures (amd64/aarch64/riscv64); BLOCK/BUFFER touches exercised
live from the REPL with no crash; a 22,000-distinct-block flood loop
against an artificially shrunk Stadium ran clean under heavy admission
load. A live MIGRATE console fire was not directly observed this
session (root-caused to a pre-existing reservoir-floor/density-eviction
interaction unrelated to this change, documented in FABRIC-3.md) --
flagged as an honest follow-up, not silently claimed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CXjAPTEKrgY2Mrk25KoLDn

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