feat(vector/search): Stage-2 rerank for Flat/IVF via the shared pipeline (#650 PR-2)

[mosuka]

Summary

PR-2 of the #650 rerank-pipeline campaign: Stage-2 rerank (Issue #481) now works end-to-end on Flat and IVF through the PR-1 RerankPipeline — previously both returned NotImplemented for rerank_factor, with all three layers (emit / load / consume) missing.

Changes

• Config wiring gap (found by the e2e test): FlatIndexConfig/IvfIndexConfig always had a rerank_storage field, but VectorStore::extract_index_type_config never mapped it from the schema option (only HNSW's from_hnsw_option did) — so nothing downstream could ever fire. Both arms now map it (the bug(vector/store): HnswOption::rerank_storage is never propagated to HnswIndexConfig — Stage-2 sidecar is not emitted via VectorStore/Engine #790 defect class: an option dropped at the config seam).
• Writers: mirror HNSW's sidecar block — temp+rename atomicity, payload in each writer's exact record-emission order (Flat: pre-sorted self.vectors; IVF: the cluster-grouped flatten order via the already-materialized all_vectors) so the reader's position mapping is the identity. Also removed a pre-existing duplicated comment in the IVF writer.
• Readers: new shared rerank_sidecar::load_rerank_sidecar helper (generalized from the HNSW reader's inline block): Eager-only, lenient-if-absent, loud on dim/vector_count mismatch. Both readers gain the pool field + accessor.
• Searchers: the upfront NotImplemented is gone. Field-routed queries with a loaded sidecar run the sorted quantized candidates through a one-stage RerankPipeline (F32SidecarStage); survivors carry exact f32 distances and stamp score_basis = "f32-rerank" (bug(vector/search): cross-segment scores are not comparable — out-of-range queries clamp to similarity 1.0 per segment #927 fan-out contract — segmented layouts behave correctly for free). Otherwise rerank_factor silently falls back to Stage 1, the same convention as HNSW's missing-sidecar path. include_vectors refetches vectors only for the final top-k on the rerank path.
• Docs (EN+JA, 6 files): the "Flat / IVF … do not yet write or consume the sidecar" caveats replaced with three-index-type support notes (schema_format.md, grpc_api.md, vector_indexing.md).

Tests

• New vector_rerank_flat_ivf_test.rs (4 e2e mirrors of the HNSW suite): sidecar emitted at segment_00000N.{flat,ivf}.f32, doc1 wins, rerank score differs bit-wise from the int8 score (proving the f32 pool was used); the silent Stage-1 fallback on sidecar-less fields now succeeds (pinning the NotImplemented removal); a multi-commit segmented Flat case exercises the fan-out's exact-basis skip.
• RED-proved: disabling the searcher-side consumption fails the "score changed by rerank" halves; restore is green.
• The IVF record-order/sidecar-order identity is verified empirically by the exact-score assertions (a mapping skew would rescore against the wrong vectors and fail them).

Verification

• cargo fmt --check and cargo clippy --all-targets -- -D warnings clean on stable and 1.97.0
• cargo test -p laurus --lib: 1261 passed; --tests: all 67 binaries ok
• wasm32 cargo check -p laurus-wasm: clean; cargo doc --no-deps -p laurus: 73 warnings (baseline)
• markdownlint-cli2: 0 errors; both mdBooks build

Closes #932. Part of #650; depends on #931 (merged as #935). Leaves #673 (PR-3: SQ sidecar + 3-stage chain) as the campaign's remaining child.