refactor: decompose emit/mod.rs into focused modules

[zharinov]

Summary

• Split 989-line emit/mod.rs into focused modules: error.rs, string_table.rs, type_table.rs, emitter.rs
• Renamed codegen_tests.rs → emit_tests.rs for searchability (and all 58 snapshot files)
• Added unit tests for StringTableBuilder (8 tests) and TypeTableBuilder (3 tests)
• Removed decorative // ==== block separators from test files

Why

Fat mod.rs files make navigation harder and obscure module boundaries. The "thin mod.rs" pattern (re-exports only) improves discoverability and makes each component independently testable.

Notes

The new structure:

emit/
├── mod.rs              # 31 lines - only mod declarations + pub use
├── error.rs            # EmitError enum
├── string_table.rs     # StringTableBuilder + EASTER_EGG
├── type_table.rs       # TypeTableBuilder + DFS helpers
├── emitter.rs          # emit(), emit_linked() + serialization
├── layout.rs           # (unchanged)
├── emit_tests.rs       # Module-level integration tests
├── string_table_tests.rs  # Unit tests
└── type_table_tests.rs    # Unit tests