Ship type declarations for @webjsdev/server (fix TS7016 on import)

[vivek7405]

Problem

@webjsdev/core ships an index.d.ts overlay, but @webjsdev/server ships no .d.ts and no types export condition in its package.json. So under strict + nodenext, a TypeScript app's import { createRequestHandler, cors, cache, createAuth, rateLimit, sitemap, validateInput, revalidatePath, ... } from '@webjsdev/server' emits TS7016 ("Could not find a declaration file for module '@webjsdev/server'").

This was surfaced while building #265 (the webjs typecheck command) and #267 (the handle() test harness): it is exactly why #265 deliberately did NOT wire npm run typecheck into the scaffold CI as a hard gate, because a fresh scaffold would fail it on the server import. It also means the Next.js migration guide (#273) and the recipes (#272) show @webjsdev/server imports that are currently untyped, and every TS webjs app gets any/an error for a first-party module the docs push as the API.

Design / approach

Add a types-only overlay for the public @webjsdev/server surface, the same posture @webjsdev/core uses (index.d.ts re-exporting from per-file .d.ts, erased at runtime, zero build cost). Type what packages/server/index.js actually exports, matching the runtime, never an idealized superset. Candidates: createRequestHandler / startServer, cors, cache / revalidateTag / revalidatePath / revalidateAll, createAuth / Credentials / Session, rateLimit, setStore / redisStore / memoryStore, sitemap / sitemapIndex, generateRouteTypes, the @webjsdev/server/testing helpers, and the @webjsdev/server/check subpath. Add a types condition to each exports subpath in package.json so editors and tsc resolve it. Keep it in lockstep with index.js (a drift guard test, like the #259 WebjsConfig schema/type lockstep).

Acceptance criteria

• @webjsdev/server ships a types overlay so a strict TS app's import ... from '@webjsdev/server' no longer emits TS7016
• The package.json exports carry a types condition for the main entry and each subpath (/testing, /check)
• The typed surface matches what index.js actually exports (a drift test asserts no missing/extra export), never a superset
• Once shipped, npm run typecheck can be wired into the scaffold CI without failing on the server import (revisit Add a webjs typecheck command (tsc --noEmit wrapper) #265's deferred CI gate)
• Tests cover the new behaviour (a tsc --noEmit fixture importing the server surface compiles clean)
• Docs / AGENTS.md updated if the public surface changed

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_{Surfaced from the production-readiness work (#265 typecheck, #267 test harness). Theme: types. Priority: P1. Kept to webjs identity: no-build, types-only overlay, erased at runtime.}