build: link bundled-dep archives / fall back to source when prebuilt is incomplete

[jonasvanderhaegen]

Problem

Building against the prebuilt liblbug.a (the default fast path in build.rs) can fail at the final link on macOS and Linux:

Undefined symbols for architecture arm64:
  "_yyjson_val_mut_copy", referenced from: lbug::json_extension::... in liblbug.a(json_utils.cpp.o)
ld: symbol(s) not found

This happens when the prebuilt liblbug.a is a thin archive — it contains objects (e.g. json_utils.cpp.o) that reference bundled third-party libraries (yyjson, simsimd, brotli, lz4, mbedtls, …) but does not fold them in, and build.rs emits no link directives for them on the prebuilt path.

Fix

link_prebuilt_bundled_deps() emits cargo:rustc-link-lib=static={lib} for each bundled dep archive shipped alongside the prebuilt liblbug.a. Guarded by exists(), so it's a no-op when an archive isn't present.

Notes / refinement since first opening

While integrating this in a downstream consumer we found two things worth flagging:

1. Use regular static linking, not +whole-archive, for the dep archives. main now bundles the third-party archives into liblbug.a via cmake/BundleStaticLibrary.cmake (a fat archive). If a consumer links both the fat liblbug.a and the individual deps/*.a with +whole-archive, the link fails with duplicate symbols on macOS/Linux. Plain static={lib} resolves the references lazily and is a no-op against a fat archive, so it works for both thin and fat layouts.

2. The undefined-symbols problem is largely moot on main now that BundleStaticLibrary produces a self-contained liblbug.a. This PR is primarily useful for consumers of older/thin prebuilts or alternate packaging that ships deps/ separately.

Verified on macOS arm64 (consuming a thin prebuilt + deps/): indexing (COPY + HNSW + checkpoint) and vector search both build and run cleanly.

[adsharma]

This will be fixed when 0.17.0 is out. For now, use

LBUG_PRECOMPILED_RUN_ID= 26256190740 when you cargo build

CI picks up the most recent ladybug nightly build which has this fixed.