feat: add streaming fast-build output

[janechu]

Pull Request

📖 Description

• Adds simulated streaming HTML chunk rendering in microsoft-fast-build, including custom element shadow template chunks, content binding splits, and stream/non-stream parity tests.
• Exposes streaming through WASM and fast build --stream, writing raw HTML chunks to stdout without file output or the normal Built: message.
• Documents streaming behavior, gotchas, and usage in crate/package docs, with a minor change file for @microsoft/fast-build.

👩‍💻 Reviewer Notes

Please focus on chunk boundaries around content bindings, custom elements, and entry-level custom element light DOM parity.

📑 Test Plan

• npm run build ✅
• cargo test --manifest-path crates\microsoft-fast-build\Cargo.toml --quiet ✅
• npm run test:node -w @microsoft/fast-build ✅
• npm run checkchange ✅
• WASM export verified locally: packages\fast-build\wasm\microsoft_fast_build.js and .d.ts include render_entry_stream_with_templates ✅
• npm run test was attempted; it failed in unrelated @microsoft/fast-test-harness Firefox Playwright tests with timeouts and NS_ERROR_SOCKET_ADDRESS_IN_USE while the focused fast-build validations passed.
• npm run biome:check was attempted; on this Windows checkout, biome-changed exits 1 without diagnostics due the Windows .bin\biome shim resolution/local Git EOL warning. Manual equivalent node_modules\.bin\biome.cmd check <changed files> passed.

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.