perf(vector/format): LVS v3 header with per-segment field-name dictionary (#857)

[mosuka]

Summary

Format-layer half of the #633 per-segment field-name dictionary (PR-A1 of the plan posted there). Inert by design: no writer stamps version 3 yet, so no bytes on disk change — this PR only teaches the shared VectorSegmentHeader the v3 vocabulary so the reader/writer migration (#858) can land as a focused follow-up.

• VERSION_FIELD_DICT = 3 / MAX_SUPPORTED_VERSION = 3; the monotone version ladder (1 = base, ≥2 = HNSW ordinal graph from perf(vector/hnsw): segment-local u32 ordinal graph + LVS1 v2 format (sub-issue of #686) #853, ≥3 = dictionary records) is documented on CURRENT_VERSION.
• VectorSegmentHeader.field_dict: Vec<String> + with_field_dict(); serialized_size / write_to / read_from carry the dictionary block only at version ≥ 3 — v1/v2 headers stay byte-identical (asserted by test).
• Dictionary allocations are self-bounded by the u16 length domain (≤ 65,535 entries × ≤ 64 KiB names), preserving the reliability(vector/index): bound reader allocations sized from unverified on-disk header counts (generalize #791) #806 no-huge-allocation property with no file-size plumbing; writers reject oversized dicts/names with InvalidOperation.
• Shared helpers consumed by perf(vector/index): read + write LVS v3 field-dictionary records in HNSW/Flat/IVF (sub-issue of #633) #858 (marked #[allow(dead_code)] until then): read_record_field() (v3 = u16 id + dictionary bounds check → LaurusError::index on corruption; legacy = the existing inline u32+UTF-8 parse), record_prefix_size() (10 vs 12), build_field_dict() (first-appearance order).

Testing

• 8 new format tests: v3 roundtrip with 0/1/3-entry dicts (serialized_size == written bytes), v1 stays 24 bytes with an empty dict, truncated dict → Io, oversized-name write rejection, read_record_field v3 resolve + out-of-range + legacy inline path, record_prefix_size ladder, build_field_dict first-appearance order.
• All 16 pre-existing format tests pass unmodified; lib 1212/1212; fmt / clippy (1.95 and 1.97 parity, workspace + wasm target) / cargo check wasm32 all clean.

Closes #857
Refs #633