docs(maps-test): add API section documenting run_tests entrypoint

Test-libs expose a single public function (run_tests) per ADR-0037.
Documenting it under ## API allows --lint to pass cleanly on test-libs,
now that parse_readme_api supports namespace-less H3 headers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -75,6 +75,13 @@ Sporel.exe --libs-dir=<libs-path> --test-libs=lib-core.maps-test
 
 Output is TAP version 13 on stdout; exit-code = failure-count (0 = all pass).
 
+## API
+
+### `run_tests(ctx)`
+**Syntax:** `M.run_tests(ctx) → void` (test-lib entrypoint per ADR-0037)
+
+Executes the test-suite against `lib-core.maps`. Calls `engine.exit(engine.test.failures())` at end; exit-code = failure-count.
+
 ## References
 
 - ADR-0037 (Tests-as-Libs Convention)