feat(core-eslint): add no-undeclared-rate-limit conformance rule

Adds the `no-undeclared-rate-limit` ESLint rule (warn severity) that
enforces rate-limit drift at lint time:

- Warns when rateLimit.consume("X", _) is called inside a use-case but
  "X" is absent from manifest.useCases[name].rateLimit
- Warns when a declared rateLimit budget has no matching consume call
  in the use-case body (unusedDeclaration)
- Is a no-op outside use-case files

Extends _manifest-ast.js to extract the rateLimit[] field from both
parseManifestUseCases and parseManifestFully. Updates _manifest-ast
tests to include the new field in expected shapes. Registers the rule
at warn severity in plugin.js and base.js. Adds RuleTester fixtures
for all four cases (declared+matching, undeclared, unused, non-use-case).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-eslint/base.js

@@ -54,6 +54,7 @@ export default [
       "conformance/atomic-tier-import-direction": "warn",
       "conformance/pii-declaration-must-be-complete": "warn",
       "conformance/no-undeclared-consent-check": ["warn", { repoRoot }],
+      "conformance/no-undeclared-rate-limit": ["warn", { repoRoot }],
     },
   },
   {