[cschanhniem]

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

What

• schema_test_coverage.py: check_schema_test_coverage() + format_schema_coverage_report() + SchemaCoverageReport + SchemaCoverageEntry
• _find_test_references(): scans test files for schema name (including hyphen alias)
• scripts/check_schema_test_coverage.py: runnable check with --warn-only flag
• 63 tests; BASELINE 7465 to 7528

Why

The repo has 28 schemas. Without a cross-reference check, schemas can be added or modified without any test being updated — the gap is silent. This checker surfaces uncovered schemas so maintainers can add tests or document why a schema has no test.

Disconfirming pass

• The checker does NOT enforce coverage (it's advisory with --warn-only default); enforcing it would block adding schemas that intentionally don't have direct unit tests
• Schema name matching is case-insensitive and handles underscore/hyphen aliases
• No safety/release policy changed; no calibration policy touched