Add CpSharedL2PageAllocator.placement_digest() -- a deterministic, order-independent SHA-256 of the full replicated allocator state (free list + per-object ranges + committed set). This is the B1 proof-obligation gate from the 2b design (sec 2.4, Theorem 1): the write-through (2b.1) cross-rank assert hashes this each tick and EQ-checks it across CP ranks, turning placement-determinism into a fail-loud runtime invariant -- any reserve/release fired on a subset of ranks or with a per-rank arg changes the digest.
6 unit tests (pure-Python, no torch): determinism, changes-on-reserve, identical-op-sequence => identical-digest, divergent-op => different-digest, reserve+release restores digest (free-list coalescing), placement-order sensitivity. Full suite 81/81 green on g0033 syh-dev-new.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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