Plugins extend explicitly matched builds

Explicit --plugin paths load Unix shared libraries for codecs, encoders, lowering providers and continuation providers on the supported LARQL commands. The host checks a compiler-and-commit ABI stamp before calling Rust-typed registration. Unknown commits and mismatches refuse; there is no automatic discovery. Duplicate identities refuse, and lowered Metal arms cannot use plugin lowering overrides. Codec support, execution support and behavioral approval are separate claims.