Suppress hydration-only reader reindexes

[Shengyu Fu (shengyfu)]

Summary

• persist a domain-versioned BLAKE3-128 identity for the exact decoded bytes used to build each text file's postings, as optional compact c evidence in filestamps.json
• suppress only forced reader-backed events whose freshly decoded identity still matches trusted evidence, while preserving byte/version verification, cache invalidation, stamp refresh, filename membership repair, and all live overlay/tombstone behavior
• propagate identities through full, delta, stale, live, and checkpointed background builds, and make direct, staged, rollback, and empty-index publication generation-safe
• preserve the public FileStamp { mtime, size } API/equality and legacy JSON compatibility; missing or malformed identities fail open

Stack

#130 is merged. This PR now targets main and remains the upper member of stack #133. It leaves #130's exact raw-byte + overlay-ID duplicate suppression unchanged.

Validation

• mutation control: disabling content-ID matching makes the reader-backed suppression test fail with a new live overlay and reindex: modified
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo build --release --workspace

Fixes #131