Removes the per-entity handoff from the freeze entirely. GenericEntityPersistence
publishes 4096-slot ranges over its dictionary's backing entries array, and workers
serialize occupied slots (value != null) directly via a ShadowEntry<TValue> struct
that mirrors the runtime's private Dictionary Entry layout. Safe because the
dictionary is frozen during Saving (mutations divert to the pending safety queues).
The layout is proven at startup before any code reads through it: validation
measures the true Entry stride via precise allocation accounting (so all shadow
reads are guaranteed in-bounds), then compares every key and value of a churned,
resized, freelist-exercised dictionary reading value slots as raw pointer bits
only - never materializing a managed reference until the layout is proven. If a
future runtime changes Dictionary internals, validation fails and saves fall back
to the enumerate-and-push path with a logged warning.
The main thread now joins the drain through an inline (threadless) worker after
publishing work, instead of idling while the thread workers finish - worth a full
worker share on the freeze and proportionally more on low-core hosts.
Measured through the real pipeline classes (24 cores, 10M entities, 1.7GB, dense
2-byte write profile): publish cost drops from ~55ms to ~0.1ms, the freeze is now
bound by pure serialize throughput at ~99ms steady state (vs ~740ms before this
branch, ~7.5x), and the first save after boot drops from ~468ms to ~139ms because
fine-grained ranges self-balance without needing size estimates.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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