refactor(antigravity): route replay merge through the request index

The indexing change left two parallel implementations of the same replay
matching logic: the filter path used the request index while the merge and
insert paths still rescanned the payload. Filter and merge must agree on which
part a ledger item targets, so the duplication was a latent source of signatures
being replayed onto the wrong part.

Move the write path onto the index. Every lookup in the merge path already ran
before the first mutation, so one index describes the whole call; insert rebuilds
it after each mutation to keep sequential semantics. Collapse the thought
signature locator into thoughtSignaturePartIndex, shared by the eligibility check
and the write path.

Delete the superseded payload-scanning implementations along with functions that
had no production callers left: antigravityNeedsSignatureReplayForExistingFunctionCall,
antigravityRequestHasMatchingFunctionResponse, antigravityPayloadHasFunctionCallID,
filterAntigravityReasoningReplayItemsForRequest, insertAntigravityReasoningReplayItems,
mergeAntigravityFunctionCallPartReplay and antigravitySetReplayItemContextHash.

Freeze the pre-index implementations into a dedicated oracle test file. The
differential tests previously called the production functions they were meant to
check, so consolidating the logic would have silently turned them into
self-comparisons.

Malformed non-array parts now fail closed consistently. gjson's Result.Array()
yields a one-element slice for a non-array non-null value, so the old fallback
scans could match a functionCall inside a parts object while the primary ID
lookup could not. The end state was already identical because such a payload
cannot be written to; the behavior is pinned by a test.

Also cover the positional fallback for pre-targetHash cache entries, which had
no test at all, and add a write-path benchmark.