[cschanhniem]

Implements J9 from NEXT_100_PR_MAP: cross-reference checker between schemas and tests.

What

• src/openamp_foundry/checks/schema_test_coverage.py: STC- coverage report
• SCHEMA_SOURCE_DIRS (9 dirs), IGNORED_MODULES (init, conftest)
• SchemaTestCoverageEntry (5 fields), SchemaTestCoverageReport (8 fields)
• check_schema_test_coverage(repo_root, source_dirs): scans each source dir, finds expected test path, checks existence
• format_schema_test_coverage_report(): text report with uncovered module list
• 63 tests across 4 classes including real-repo integration test
• BASELINE 8158→8221

Why

Without a cross-reference checker, schema modules can be added without test files and the gap is invisible until tests break. The STC- report shows exactly which schema modules lack test coverage, making schema→test coupling enforceable in CI.

Disconfirming pass

• init.py and conftest.py are excluded from scanning
• Nonexistent source dirs are skipped (no error)
• Empty source dirs return is_fully_covered=True (correct: no schemas → no gap)
• Real-repo integration test asserts coverage_fraction >= 0.5 (not 1.0, since legacy modules may lack test files)