Introduces circuitforge_core.config.license with validate_license() and
get_license_tier(). Both functions are safe to call when CF_LICENSE_KEY
is absent, returning free tier gracefully. Results are cached 30 min per
(key, product) pair. CF_LICENSE_URL env var overrides the default
Heimdall endpoint. Re-exports added to config.__init__. Existing
test_config.py moved into tests/test_config/ package to co-locate with
new test_license.py (10 tests; 204 total passing).