perf(vector/index): use `Vec<u32>` doc-id internal type throughout the index (instead of `u64`)

[mosuka]

Round-3 perf push sub-issue (tracked under umbrella #535).

[S] Use Vec<u32> doc-id internal type throughout the index (instead of u64)

• Where: HNSW graph (graph.rs:21-22,73), Concurrent build
  (writer.rs:40), QuantizedVectorPool::field_index value type
  (quantized_storage.rs:45), neighbour lists, etc.
• Current behaviour: doc IDs are u64 end-to-end inside the
  index, even though they're segment-local ordinals once we get past
  the public API.
• Why it might be a bottleneck: every neighbour list, visited
  set, candidate heap stores 8 B per entry — half what is needed
  for <=2^31 segment sizes (segment size is anyway capped by the
  vector_count u32 in IVF/Flat headers).
• Reference precedent: Lucene uses int ordinals; Qdrant uses
  u32 PointOffset internally.
• Suggested direction: introduce InternalId = u32 with From/
  TryFrom<u64> conversion at the boundary; refactor graph storage
  • visited set to u32 (already started for perf(vector/hnsw): replace HashSet visited-set with bitmap #406 visited bitmap).
    Single-segment caps at 4 G vectors which is fine.
• Risk / scope: small but pervasive type churn; pairs naturally
  with the CSR migration (Add parallel search #1).

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ID: VI-17 — see ~/.claude/tasks/laurus/20260523_perf_round3_audit/task_list.md for the full Round-3 issue list.

[mosuka]

Closing: this is the earlier sibling of #686, and its premise was settled there. A global u32 doc-id type is unsound in laurus — ids are engine-global, monotonic, shared with the lexical index, and the lexical shard scheme sets the upper 16 bits (util/id.rs), so any u64→u32 cast breaks. The sound form (segment-local u32 ordinals + ordinal→u64 table) shipped for the read side in #853/PR #854 (−56..−86% on the store-level gates); writer build internals deliberately stay u64-keyed because the active-segment search path is a flat scan (rationale recorded in #853). Nothing actionable remains under this issue's framing.