codexReasoningReplayTurnAnchorIndex re-hashed the entire input prefix for
every probed index while scanning anchors downward, turning the anchor
search into O(n^2) SHA-256 over the conversation. On long-context (1M)
sessions the gateway spent minutes hashing per request before contacting
upstream, which surfaced as client-side 'operation timed out' errors and
~11 cores of steady CPU on the gorillaclaw candidate.
Replace the per-probe recomputation with a lazily extended incremental
digest that snapshots each prefix sum once, keeping every probe O(1)
after a single O(n) pass. Output is bit-identical to
codexReplayInputPrefixFingerprint; equivalence covered by a new test.
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