perf(vector/search): HNSW prefetch is x86_64 + aarch64 only; on wasm32 the search path takes a hit

[mosuka]

Round-3 perf push sub-issue (tracked under umbrella #536).

[S] HNSW prefetch is x86_64 + aarch64 only; on wasm32 the search path takes a hit

• Where: laurus/src/vector/index/hnsw/searcher.rs:617-633 — #[cfg(target_arch = "x86_64")] / aarch64 blocks; wasm32 silently no-ops the inner branch.
• Current behavior: WASM build runs the prefetch loop body for no benefit (just the
  HashMap lookup and an inner loop that does nothing).
• Suggested direction: Wrap the entire prefetch pass behind #[cfg(not(target_arch = "wasm32"))]. Compile out the field_prefetch index entirely on WASM.
• Risk / scope: Trivial. Just cfg gating.

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ID: VS-14 — see ~/.claude/tasks/laurus/20260523_perf_round3_audit/task_list.md for the full Round-3 issue list.