feat(p2-shim): add buffered input stream helper for browser cli stdin

[eduardomourar]

Summary

Every host feeding a guest a fixed stdin buffer hand-rolls the same bounded InputStreamHandler, and it's easy to get the "closed" transition or u64 length truncation subtly wrong.

Changes

• Adds bufferedInputStream(bytes) in a new src/browser/helpers/cli.ts, so hosts can pass it straight to createCli({ stdin }) (or _setStdin) instead of hand-rolling it.
• The helper is browser-only (it builds on the browser io pollables), so it is exposed from an explicit browser/helpers/cli subpath export rather than the platform-neutral ./* wildcard. This keeps convenience helpers separate from the shim's implementation and the underscore-prefixed configuration surface, and makes the browser-only nature visible in the import path:

import { bufferedInputStream } from "@bytecodealliance/preview2-shim/browser/helpers/cli";

• Documents the helper in a new Jco book page, Preview 2 shim helpers → CLI helpers (docs/src/advanced/preview2-shim-helpers/cli.md), covering createCli, WASIShim, and the global setter. The package README only mentions the helper and links to that page. feat(p2-shim): add host-side write/read helpers for browser preopens #2237 adds a sibling filesystem.md under the same group, so whichever lands second has a one-line SUMMARY.md conflict.

Node consumers now get the Node typings

The . and ./* exports had a top-level types pointing at the browser .d.ts ahead of the node block. TypeScript under nodenext includes node in its condition set but stops at the first matching condition, so Node consumers were typed against the browser build. The types entries are now nested under node and default with no top-level types key, which oxfmt's package.json sorting leaves alone. Verified with a consumer project: moduleResolution: nodenext resolves dist/nodejs/*.d.ts, moduleResolution: bundler resolves dist/browser/*.d.ts, and Node runtime resolution is unchanged. The same hunk is in #2237 so the two merge cleanly in either order.

[vados-cosmonic]

LGTM 🚀

[vados-cosmonic]

Hey @eduardomourar on second thought here -- would you mind moving this helper actually to a different file and exporting them from a new export like ...helpers/cli?

I want to improve the DX here like you have done but I'd like to separate the helpers from the actual implementation-specific code

[jco-release-bot]

🎉 The code in this PR was made available in the following release:
https://github.com/bytecodealliance/jco/releases/tag/preview2-shim-v0.29.0