Add M1 selector intersections

[sabino]

Summary

• add dbt-style comma intersection for the implemented selector subset
• apply graph expansion per selector term so expressions such as customers+,config.materialized:table intersect correctly
• add model-only config.materialized:<value> selection for resolved model materializations
• reject unsupported selector methods, empty comma terms, empty config values, whitespace unions, and repeated graph operators
• update selector docs/status notes and fixture coverage

Validation

• zig fmt --check src/project.zig src/root.zig
• zig build test --cache-dir /tmp/dxt-zig-cache-local --global-cache-dir /tmp/dxt-zig-cache-global
• pytest -q (39 passed)
• zig build -Doptimize=ReleaseSafe --cache-dir /tmp/dxt-zig-cache-local --global-cache-dir /tmp/dxt-zig-cache-global
• ReleaseSafe selector smoke: dxt ls --project-dir tests/fixtures/selector_graph --output json --select 'customers+,config.materialized:table'
• python scripts/check_public_safety.py
• python scripts/check_runtime_boundary.py
• git diff --check

Agent Review

• planning agent reviewed dbt selector intersection semantics and recommended per-term graph expansion plus model-only config.materialized
• final review agent found no blockers; repeated + validation was tightened before commit