fix(pow): mark Node-only imports so browser bundlers skip them

[0x3639]

Summary

Follow-up to #34 (targets feature/browser-bundles, one commit). Prefixes the three Node-only dynamic imports in src/pow/pow.ts with node: and adds /* @vite-ignore */ alongside the existing /* webpackIgnore: true */:

const { readFileSync } = await import(/* webpackIgnore: true */ /* @vite-ignore */ "node:fs");
const { fileURLToPath } = await import(/* webpackIgnore: true */ /* @vite-ignore */ "node:url");
const { dirname, join } = await import(/* webpackIgnore: true */ /* @vite-ignore */ "node:path");

Why

With the modular build now resolved directly by browser bundlers, Vite/Rolldown still tries to resolve the bare fs/url/path specifiers even though they sit under isNode():

[plugin rolldown:vite-resolve] Module "path" has been externalized for browser compatibility, imported by ".../znn-typescript-sdk/dist/pow/pow.js"
[plugin rolldown:vite-resolve] Module "fs" has been externalized for browser compatibility, imported by ".../znn-typescript-sdk/dist/pow/pow.js"

and, when the consumer has a Node polyfill plugin (common for Buffer), it emits a never-fetched url polyfill chunk (~51 KB minified) referenced from the SDK chunk. The node: prefix plus @vite-ignore makes every bundler leave the specifiers alone.

Verification

• zwap.fun (Vite 8.1.4 / Rolldown, production): both warnings gone, url chunk gone, 271 → 223 modules transformed, no __vite-browser-external stubs emitted. Tree-shaking results from feat: make the browser build tree-shakeable, drop UMD #34 unchanged (no argon2 / KeyFile / KeyStore / eval in output).
• esbuild 0.28 --bundle --platform=browser on dist/index.js: clean exit.
• Node 22.12 from the packed tarball: PoW init() + generate() succeed through the node: imports; KeyStore / KeyFile round trip unaffected.
• npm test: 650 passing. npm run lint / npm run build clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NoKrSzV9FwJHGutEwsATAY