Support file selectors

[sabino]

Summary

• Add file: selector validation and matching for selectable resources through the shared Zig selector engine.
• Match dbt Core-style original_file_path basename and stem values such as file:orders.sql, file:orders, and file:*orders.sql without matching path components.
• Cover model SQL, seed CSV, generic-test schema YAML, source schema YAML, exposure schema YAML, and docs generate selector reuse.
• Update the selector compatibility docs, README snapshot, changelog, PLAN, and source-grounded research note.

Validation

• codex exec --profile azure -m gpt-5.5 -s read-only PR-boundary review: no remaining findings after follow-up.
• zig fmt --check src/root.zig src/project/selector.zig
• zig test src/project/selector.zig --cache-dir /tmp/dxt-zig-cache-selector-file --global-cache-dir /tmp/dxt-zig-global-cache
• zig test src/root.zig --cache-dir /tmp/dxt-zig-cache-root-file --global-cache-dir /tmp/dxt-zig-global-cache
• zig build --cache-dir /tmp/dxt-zig-build-cache-file-selector --global-cache-dir /tmp/dxt-zig-global-cache
• zig build test --cache-dir /tmp/dxt-zig-build-cache-file-selector-test --global-cache-dir /tmp/dxt-zig-global-cache
• pytest -q tests/test_cli.py -k "model_properties or seed_ref_dependency or resource_type_selectors_for_sources_and_exposures or docs_generate_applies_select or docs_generate_reuses_file_selectors or ls_multi_argv"
• python scripts/check_runtime_boundary.py
• python scripts/check_public_safety.py
• git diff --check