feat(core-shared): add rate-limit type primitives and manifest field

Adds IRateLimit, RateLimitBudget, RateLimitDecision to a new
rate-limit sub-module; adds RateLimited<F> brand and isRateLimited
predicate following the Captured/ConsentChecked pattern; extends
UseCaseManifest with rateLimit?: readonly RateLimitBudget[] so
features can declare rate-limit gates in their manifests.

Exports new types from @repo/core-shared/rate-limit and
@repo/core-shared/conformance. Blocks stories 02, 03, and 04.

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

packages/core-shared/package.json

@@ -6,6 +6,7 @@
   "exports": {
     ".": "./src/index.ts",
     "./audit": "./src/audit/index.ts",
+    "./rate-limit": "./src/rate-limit/index.ts",
     "./conformance": "./src/conformance/index.ts",
     "./conformance/coverage": "./src/conformance/coverage.ts",
     "./di": "./src/di/index.ts",