Add source table identifier support

[sabino]

Summary

• add table-level source identifier parsing while keeping logical source table name for unique IDs, selectors, FQNs, and dependency keys
• render source physical identifiers through compile relation names, manifest identifier / relation_name, DuckDB source catalog lookup, source freshness SQL, and source generic-test relations
• document the source-grounded slice in PLAN, compatibility docs, changelog, and research notes

Validation

• zig test src/project/parse.zig --cache-dir /tmp/dxt-zig-cache-source-id --global-cache-dir /tmp/dxt-zig-global-cache-source-id
• zig test src/project/compiler.zig --cache-dir /tmp/dxt-zig-cache-source-id-compiler --global-cache-dir /tmp/dxt-zig-global-cache-source-id-compiler
• zig test src/project/manifest.zig --cache-dir /tmp/dxt-zig-cache-source-id-manifest --global-cache-dir /tmp/dxt-zig-global-cache-source-id-manifest
• zig test src/project/duckdb.zig --cache-dir /tmp/dxt-zig-cache-source-id-duckdb --global-cache-dir /tmp/dxt-zig-global-cache-source-id-duckdb
• zig test src/project/duckdb.zig --cache-dir /tmp/dxt-zig-cache-source-id-duckdb2 --global-cache-dir /tmp/dxt-zig-global-cache-source-id-duckdb2
• pytest -q tests/test_cli.py::test_source_identifier_compiles_physical_relation_and_preserves_logical_source_key tests/test_cli.py::test_source_identifier_drives_duckdb_catalog_and_freshness_relations
• pytest -q tests/test_cli.py::test_build_executes_selected_duckdb_source_column_generic_tests
• zig fmt --check src/project/types.zig src/project/compiler.zig src/project/parse.zig src/project/manifest.zig src/project/duckdb.zig
• zig build --cache-dir /tmp/dxt-zig-cache-source-id-build --global-cache-dir /tmp/dxt-zig-global-cache-source-id-build
• zig build test --cache-dir /tmp/dxt-zig-cache-source-id-buildtest --global-cache-dir /tmp/dxt-zig-global-cache-source-id-buildtest
• python scripts/check_runtime_boundary.py
• python scripts/check_public_safety.py
• pytest -q tests/test_cli.py (115 passed in 684.12s before the final source-generic-test coverage amendment; amended commit is covered by targeted local tests and green GitHub CI)
• git diff --check origin/main...HEAD

Review

• PR-boundary Codex review found one low test coverage gap for source generic-test SQL; fixed in the amended commit by covering physical source identifiers in native DuckDB generic-test SQL and CLI source-column build coverage.