ffi: Add extensible C search parameters

[shyjsarah]

Summary

Add an append-only C search-parameter structure so future query tuning fields can be exposed without adding a new by-value ABI version for every field.

This also exposes max_initial_filter_expansion_factor to C and C++ while preserving the existing V1/V2 symbols and behavior.

Changes

• Add pointer-based PaimonVindexSearchParamsEx with a caller-provided struct_size.
  • Missing fields from shorter caller structs use existing defaults.
  • Unknown trailing fields from newer caller structs are ignored.
• Add *_ex variants for single, filtered, batch, and filtered-batch C searches.
• Keep all existing by-value C search functions unchanged for binary compatibility.
• Route the C++ RAII reader through the extensible API and expose:
  • max_initial_filter_expansion_factor
  • IVF-PQ batch table reuse mode
  • IVF-PQ batch table reuse memory budget
• Document the append-only ABI contract and C/C++ usage.

Testing

• cargo fmt --all -- --check
• cargo clippy --all-targets --workspace --locked -- -D warnings
• cargo test --workspace --locked
• C FFI roundtrip tests, including a shorter prefix struct passed to the new API
• C++ roundtrip tests and query tuning parameter forwarding
• git diff --check

Notes

• Public API: additive.
• Existing C ABI symbols and by-value parameter structures are unchanged.
• New callers should initialize struct_size with sizeof(PaimonVindexSearchParamsEx).
• Follow-up to the query tuning added in ivfpq: Reuse query distance tables across IVF lists #67 and feat: make initial IVF filter expansion configurable #68.

[jerry-024]

+1