ffi: C ABI version and capability probes for downstream engines

[Fieldnote-Echo]

Summary

Add explicit C ABI version and capability probes so downstream engines can validate the ordvec runtime they are embedding.

This complements #87, which tracks C ABI builder/add APIs. This issue is about ecosystem compatibility and being a good neighbor to non-Rust host systems that need stable dynamic checks instead of relying on crate internals or build-time assumptions.

Current gap

The repo has a C ABI surface, but downstream vector DBs and language bindings need a clear way to ask: what ABI am I loaded against, what capabilities are present, and what compatibility rules apply?

Proposed shape

Sketch only:

uint32_t ordvec_abi_version(void);
const char *ordvec_version_string(void);
uint64_t ordvec_capabilities(void);
ordvec_status ordvec_get_last_error(char *buf, size_t len);

Capability flags could cover loaded-index search, builder/add support, subset rerank, docset search, mapped loading, and file-format probe support as those surfaces exist.

Acceptance criteria

• C header exposes ABI version and capability flags.
• Symbol names and bump rules are documented.
• A C smoke test verifies version/capability calls from outside Rust.
• Docs state what changes require an ABI version bump.
• Does not require publishing the FFI crate until the maintainer decides that surface is ready.

Non-goals

• No database-specific adapter.
• No query planner.
• No metadata/payload ownership.
• No replacement for C ABI v2: builder/add APIs #87 builder/add work.