fix(build): restore deleted extract/build.rs and align stale test/exa…

[deem0n]

The Virtual Experts merge (PR #45) left main unable to build:

1. crates/larql-vindex/src/extract/build.rs was deleted but extract/mod.rs still declares pub mod build; and re-exports build_vindex / build_vindex_resume, which are still called from larql-cli, larql-lql, the vindex tests, and the demo_features example. The roadmap (vindex M8) plans to split this file into build/{mod,...}.rs, but that refactor was never finished. Restore the file from d3a8bc6^ so the workspace compiles again; the planned split can land separately.

2. New required fields had been added to several public structs but a number of test fixtures and examples were not updated, so cargo build --features metal and cargo test --workspace failed with E0063 / E0308:

   • FullPipelineLayer.ffn_is_remote — added to 6 larql-compute examples (compare_decode, compare_formats, compare_ollama, compare_pipeline, demo_architecture, diag_decode_pipeline).
   • LoadVindexOptions.moe_remote — added to bench_expert_server and openai_demo examples.
   • LoadedModel.{unit_filter, moe_remote} plus the metal-experts-conditional metal_backend / moe_scratches / metal_ffn_layer_bufs fields, and the weights field's new OnceLock<RwLock<ModelWeights>> wrapper — fixed in tests/common/mod.rs, tests/test_expert_endpoint.rs, tests/test_http_full_routes.rs, tests/test_unit_band_utils.rs, tests/test_unit_state.rs.
   • VindexConfig.{fp4, ffn_layout} and ModelWeights.skipped_tensors in tests/test_expert_endpoint.rs.
   • MoeLayerWeights.expert_data_format plus per-expert byte-slice layout (Vec<&[u8]> instead of a single &[u8]) in the same test's local_output helper.

Result: workspace builds clean with --features metal, and 3,481 tests run (the remaining failures are runtime numerical-correctness issues in Metal kernels — unrelated to this commit, and the reason this branch exists).