Add VecSimIndex_RelabelVector to re-key a vector without re-insertion

[ofiryanai]

What

Adds a public C API VecSimIndex_RelabelVector(index, old_label, new_label) that changes a stored vector's external label in place, without re-inserting the vector or modifying the graph topology.

HNSW neighbor links reference internal ids, which a relabel leaves unchanged, so only the label↔internal-id mapping and idToMetaData[id].label are updated — O(1) per stored vector, zero graph edges touched.

Why

Enables a caller to re-key an unchanged vector cheaply instead of DeleteVector + AddVector. Motivating case (RediSearch partial updates): on an HSET that doesn't modify the vector fields, RediSearch still assigns a new internal doc id, which today forces a delete + re-insert of every vector into HNSW — churning the graph (ghost/marked-deleted nodes, repair jobs) even though the vector is identical. With relabel, the existing graph node is simply re-pointed to the new label. (This is the // TODO: use VecSimReplace noted in RediSearch src/indexer.c.)

How

• vec_sim_interface.h — non-pure virtual relabelVector with a VECSIM_RELABEL_NOT_SUPPORTED default, so index types without an implementation (e.g. SVS) are unaffected and callers can fall back to delete + add.
• BruteForce single/multi — O(1) label-map relabel. No internal lock; the caller provides mutual exclusion (matching addVector/deleteVector).
• HNSW single/multi — relabelVectorUnsafe updates labelLookup + idToMetaData[id].label; relabelVector wraps it under an exclusive indexDataGuard.
• Tiered HNSW — under flatIndexGuard + mainIndexGuard (canonical order), relabels the flat-buffer copy, rewrites any pending HNSWInsertJob.label and rekeys labelToInsertJobs (so a not-yet-ingested vector is later ingested under the new label), then relabels the HNSW backend via the lock-free variant (avoids a recursive indexDataGuard lock).

Return: 1 relabeled, 0 old_label not found, -1 not supported. Caller must guarantee new_label does not already exist.

Tests

tests/unit/test_hnsw_tiered.cpp:

• RelabelVectorInBackend — relabel a vector already ingested into HNSW.
• RelabelVectorPendingInFlat — relabel a vector still in the flat buffer with a pending insert job; verifies the buffered vector and the pending job are re-keyed and the later ingestion lands under the new label.

Both run across single/multi and float/double.

Validation note

The relabel method bodies were type-checked locally via explicit template instantiation (clean). The full gtest suite was not run locally — VecSim's tiered factory hard-includes the SVS headers, and the SVS dependency chain (LVQ / fmt) doesn't configure on macOS ARM. Please rely on CI (Linux) to build and run the new tests.

🤖 Generated with Claude Code

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

---

^{Security scan by Jit}