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/plugin.js

@@ -10,6 +10,7 @@ import componentMustHaveTest from "./rules/component-must-have-test.js";
 import atomicTierImportDirection from "./rules/atomic-tier-import-direction.js";
 import piiDeclarationMustBeComplete from "./rules/pii-declaration-must-be-complete.js";
 import noUndeclaredConsentCheck from "./rules/no-undeclared-consent-check.js";
+import noUndeclaredRateLimit from "./rules/no-undeclared-rate-limit.js";
 
 /**
  * The `@repo/core-eslint` conformance plugin. Aggregates custom rules that
@@ -39,6 +40,7 @@ const plugin = {
     "atomic-tier-import-direction": atomicTierImportDirection,
     "pii-declaration-must-be-complete": piiDeclarationMustBeComplete,
     "no-undeclared-consent-check": noUndeclaredConsentCheck,
+    "no-undeclared-rate-limit": noUndeclaredRateLimit,
   },
 };