M3 L1 symbol table: tree build, impl-facet rule, JSON emission, L1 gate

[rahlk]

Part of #27. Rs* entity types + can:// signature grammar (fully-qualified paths); tree build from IR with two-pass unified impl-facet rule; inline-mod flattening; macros{} map + from_macro attribution; call nodes (callee null) in body{}; analysis.json emission at -a 1. Gate: schema validation; relative-path keys; exact get_method_body byte-slicing; cache reuse on re-run; value-assertion tests for every Rust-specific field.