fix(kvflash): pooled prefix snapshot at chunk boundary (≥128K restore)
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DU
4mo ago+1440+1440+0123456789012345678901234567890123456789-256-256-012345678901234567890123456789
Wave-1 KVFlash deep-context. New commit in this PR: feee0a1d — save the pooled prefix snapshot at a chunk-aligned boundary so ≥128K restore works (was a crash).
Stacking / merge order: depends on #428 (pool reservation+sizing), #429 (pager serde), and #430 (pooled chunked prefill core). Those appear as lower commits in this branch's diff; review/merge them first — this branch rebases to drop them once they land, leaving only the snapshot-boundary fix.
Validation: rebased clean onto current main; the composed deep-context stack passed 64K NIAH (needle exact, restore engaged) on the integration branch. No standalone agentic gate yet.