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/rules/_manifest-ast.test.js

@@ -42,6 +42,7 @@ describe("parseManifestUseCases", () => {
         publishes: [],
         consumes: [],
         analyticsEvents: [],
+        rateLimit: [],
       },
       signUp: {
         mutates: true,
@@ -49,6 +50,7 @@ describe("parseManifestUseCases", () => {
         publishes: ["auth.signed-up"],
         consumes: [],
         analyticsEvents: [],
+        rateLimit: [],
       },
       signOut: {
         mutates: true,
@@ -56,6 +58,7 @@ describe("parseManifestUseCases", () => {
         publishes: [],
         consumes: [],
         analyticsEvents: [],
+        rateLimit: [],
       },
     });
   });
@@ -94,6 +97,7 @@ describe("parseManifestFully", () => {
           publishes: [],
           consumes: [],
           analyticsEvents: [],
+          rateLimit: [],
         },
         signUp: {
           mutates: true,
@@ -101,6 +105,7 @@ describe("parseManifestFully", () => {
           publishes: [],
           consumes: [],
           analyticsEvents: [],
+          rateLimit: [],
         },
       },
     });