perf(vector/hnsw): fold the .hnsw checksum into the Eager load pass (#789)

[mosuka]

Summary

Removes the redundant full read of .hnsw segments on the Eager load path while keeping the #786 corruption guarantee.

#786 verified the CRC-32 footer with verify_checksum_footer — an independent full pass over the content before the structural parse. In Eager mode the parse then reads the same content again, so a footer-carrying segment was read ~twice on every reader creation (every searcher-cache miss, i.e. after each commit()).

This PR folds the CRC into the single Eager structural pass and verifies the footer after parse — no extra read. The Lazy/OnDemand path (which seeks over the vector payload) keeps a dedicated up-front verification pass; legacy footer-less segments skip verification.

Changes

• storage/checksum.rs — new ChecksumTrackingInput, a StorageInput wrapper that accumulates a CRC over sequential reads. A real seek clears is_sequential; the no-op Current(0) seek used by stream_position() is served from a byte counter so it does not break tracking; track=false degrades it to a thin position-tracking pass-through; absorb_to(len) covers residual bytes; clone_input() returns the unwrapped inner (OnDemand clones inherit no running-CRC state).
• vector/index/hnsw/reader.rs — verify_checksum_footer split into read_footer_crc (8-byte footer probe only) and verify_footer_content (Lazy dedicated pass). load() sets fold = Eager && footer_present, runs the structural parse through ChecksumTrackingInput, then absorb_to(content_len) + compares against the stored CRC, falling back to verify_footer_content if is_sequential() is unexpectedly false.

Tests

• ChecksumTrackingInput unit tests (sequential CRC, stream_position keeps sequential, real seek clears it, absorb_to, track=false, clone_input unwraps inner).
• Corruption rejection on Eager (Scalar8Bit), Lazy/mmap, and pq-fastscan (feature-gated) segments.
• Legacy footer-less load on both Eager and Lazy/mmap paths.
• Read-count measurement (eager_load_reads_hnsw_segment_exactly_once): a CountingStorage asserts the Eager load reads the segment exactly file_size bytes (footer probe + one folded pass). The pre-perf(vector/hnsw): avoid the extra full read when verifying the .hnsw checksum on load #789 double-read was ~2 * content_len + 8, so this is a deterministic regression guard for the latency-restored criterion.

Verification

• cargo test -p laurus --test vector_hnsw_checksum_test → 5 passed
• cargo test -p laurus --lib checksum → 9 passed
• cargo test -p laurus --lib vector::index::hnsw → 37 passed (default) / 38 passed (--features pq-fastscan)
• cargo fmt --check clean; cargo clippy -p laurus --all-targets -- -D warnings clean (default and --features pq-fastscan)
• cargo build -p laurus-wasm --target wasm32-unknown-unknown → success

Notes

• HnswIndexReader::load signature is unchanged → no language-binding impact.
• An adversarial multi-agent review (fold-correctness, position-tracking, lazy/ondemand, back-compat, test-adequacy) found no correctness defects; the actionable coverage gaps it surfaced are included in the tests above.

Closes #789