feat(antfu): document tsnapi, tsdown-stale-guard, and fast-npm-meta

Add an API Stability section to library-development covering tsnapi +
tsdown-stale-guard as paired tsdown plugins for snapshotting public
exports and guarding against stale builds in CI. Add a fast-npm-meta
recipe under Tooling Choices for cheap latest-version lookups.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

skills/antfu/SKILL.md

@@ -3,7 +3,7 @@ name: antfu
 description: Anthony Fu's opinionated tooling and conventions for JavaScript/TypeScript projects. Use when setting up new projects, configuring ESLint/Prettier alternatives, monorepos, library publishing, or when the user mentions Anthony Fu's preferences.
 metadata:
   author: Anthony Fu
-  version: "2026.02.03"
+  version: "2026.05.01"
 ---
 
 ## Coding Practices
@@ -58,6 +58,20 @@ metadata:
 | `nci` | Clean install (`pnpm i --frozen-lockfile`) |
 | `nlx <pkg>` | Execute package (`npx`) |
 
+### Checking npm Package Versions
+
+Use [`fast-npm-meta`](https://github.com/antfu/fast-npm-meta) to look up the latest version of a package — it queries a small metadata endpoint instead of downloading the full registry payload (which can be megabytes per package).
+
+```bash
+nlx fast-npm-meta version vite              # 7.3.1
+nlx fast-npm-meta version "nuxt@^3.5"       # 3.5.22 — range-aware
+nlx fast-npm-meta version vite nuxt vue     # multiple at once
+nlx fast-npm-meta version vite --json       # JSON for scripting
+nlx fast-npm-meta full vite                 # full version list + dist-tags
+```
+
+Prefer this over `npm view <pkg> version` when you only need the latest version, and over reading `package.json` from the registry directly.
+
 ### TypeScript Config
 
 ```json

skills/antfu/references/library-development.md

@@ -52,6 +52,59 @@ The `exports: true` option auto-generates the `exports` field in `package.json`
 
 ---
 
+## API Stability
+
+For published libraries, lock the public API surface so accidental breaking changes appear as a diff in code review.
+
+| Tool | Purpose |
+|------|---------|
+| [`tsnapi`](https://github.com/antfu/tsnapi) | Snapshots runtime exports + type declarations into committed `.snapshot.js` / `.snapshot.d.ts` files |
+| [`tsdown-stale-guard`](https://github.com/antfu-collective/tsdown-stale-guard) | Hashes build inputs/outputs so CI can prove the committed snapshots match current source |
+
+Install both as dev dependencies and wire them as tsdown plugins:
+
+```ts
+// tsdown.config.ts
+import { defineConfig } from 'tsdown'
+import ApiSnapshot from 'tsnapi/rolldown'
+import { StaleGuardRecorder } from 'tsdown-stale-guard'
+
+export default defineConfig({
+  entry: ['src/index.ts'],
+  format: ['esm'],
+  dts: true,
+  exports: true,
+  plugins: [
+    ApiSnapshot(),
+    StaleGuardRecorder(),
+  ],
+})
+```
+
+### Updating snapshots
+
+After an intentional API change, regenerate snapshots and commit them:
+
+```bash
+nr build              # runs tsdown, regenerates .snapshot.* files
+# or, ad-hoc:
+nlx tsnapi -u
+UPDATE_SNAPSHOT=1 nlx tsnapi
+```
+
+### CI guard
+
+`tsdown-stale-guard` ships a CLI that exits non-zero when the cached build hash no longer matches the source — run it before snapshot/test steps to guarantee the committed snapshots reflect current code:
+
+```yaml
+# .github/workflows/unit-test.yml (excerpt)
+- run: nr build
+- run: nlx tsdown-stale-guard   # fails CI if build is stale
+- run: nr test
+```
+
+---
+
 ## package.json
 
 Required fields for pure ESM library: