feat: add core types, defineConfig, and resolvePlan

[Apetuezekiel]

What

Implements the config schema layer: types, defineConfig, and resolvePlan.

New files:

• src/types.ts — Period, Limit, PlanInput, ConfigInput, ResolvedPlan, and the generic Config<TPlanName, TFeature, TLimitKey> type used by downstream PRs
• src/config.ts — defineConfig<const T extends ConfigInput>(config: T): T identity function with const type parameter for full literal inference
• src/resolve.ts — resolvePlan(config, planName) that walks the extends chain, accumulates features (deduplicated), merges limits (descendant wins), and throws typed errors on circular or unknown extends

Updated:

• src/index.ts — exports defineConfig and all public types
• src/index.test.ts — updated from stub to verify public export surface
• vitest.config.ts — adds typecheck.include for tests/**/*.test-d.ts

Tests:

• src/config.test.ts — 4 tests covering identity return, limits, extends, null quota
• src/resolve.test.ts — 17 tests: feature accumulation at depth 0/1/2/3, dedup, limit inheritance, limit override, deepest-wins, null quota round-trip, unknown root plan, unknown extends target, circular self-reference, indirect cycle, error messages
• tests/types.test-d.ts — 3 type tests verifying literal plan name inference, extends field typing, quota number | null type

Why

Implements v1 scope items 1 and 5: the declarative plan/feature/limit schema with extends single-inheritance and full TypeScript literal-type inference from static config.

How to verify

npm run lint       # clean
npm run typecheck  # clean
npm run test       # 25 tests pass, 100% coverage on config.ts, index.ts, resolve.ts
npm run build      # dist/ emitted with ESM + CJS + types

Out of scope

• createAccess, error classes, override logic — those are PR 3
• JSON Schema generation — PR 4
• overrides.limits runtime API — deferred to v1.1