C ABI v2: builder/add APIs

[Fieldnote-Echo]

Summary

Base C ABI v1 exists as the repo-local, publish=false ordvec-ffi crate with include/ordvec.h. It covers loading persisted RankQuant and Bitmap indexes through opaque handles plus synchronous full/subset search. The Go wrapper (ordvec-go) sits on top of that surface.

This issue now tracks the next ABI layer: builder/add APIs for callers that need to construct indexes through C rather than loading persisted files.

Scope

• Add C ABI builders for supported index types.
• Add add/ingest entry points with explicit validation and status codes.
• Preserve the existing panic boundary: no Rust panic crosses the C ABI.
• Define ownership/lifetime for any new buffers or handles.
• Update the Go wrapper after the C ABI v2 contract is pinned.

Notes

Keep this repo-local for now; ordvec-ffi remains publish = false for v0.3.0.

[Fieldnote-Echo]

Base C ABI v1 now exists as the repo-local, publish=false ordvec-ffi crate with include/ordvec.h, covering persisted RankQuant/Bitmap loads and synchronous search. Retitling this issue to track the remaining v2 builder/add APIs instead of the completed base ABI.