feat: add webui-framework-prerelease conversion target with strip-and-warn directives

[Jane Chu (janechu)]

Pull Request

📖 Description

Adds a new conversion target, webui-framework-prerelease, to microsoft-fast-convert / @microsoft/fast-build, alongside the existing webui-prerelease and fast-v3-ts targets. webui-prerelease is unchanged by this PR — it continues to emit HTML for WebUI's own FAST parser plugin exactly as before.

webui-framework-prerelease converts FAST's declarative syntax into the Microsoft WebUI Framework's own declarative runtime syntax, to be leveraged by their web component framework:

• Transfers Shadow/Light DOM policy: shadowroot-prefixed attributes move from the outer <f-template> wrapper onto the inner <template>, deduping identical values and rejecting conflicting ones (ConflictingShadowRootMode) or unsupported outer attributes (UnsupportedPublisherAttribute).
• Converts f-ref="{input}" → w-ref="{input}", rejecting non-braced values (InvalidAttributeValue).
• Rewrites event handler $e to bare e and rejects $c/other FAST event-context tokens that have no equivalent (UnsupportedEventContext).
• f-slotted and f-children are stripped from the output with a warning instead of being hard errors or passed through verbatim — WebUI Framework has no equivalent for these directives, but failing the whole conversion for them was too strict. Every other unsupported construct remains a hard ConvertError.

Because conversion can now produce non-fatal warnings, convert_template/convert_template_with_options return a new ConvertOutput { output, warnings } (Rust) / ConvertResult (wasm, with output/warnings getters) instead of a bare string, for all three targets — though only webui-framework-prerelease ever returns a non-empty warnings. The fast convert CLI prints each warning to stderr as Warning: <message> and still writes the converted output; it doesn't fail the command. convert_stylesheet is unaffected and still returns a plain string.

The target's name, webui-framework-prerelease, reflects that the WebUI Framework project itself is currently in prerelease — it isn't a statement about the stability of this conversion target's implementation; that wording is intentionally not used elsewhere.

👩‍💻 Reviewer Notes

Most review attention should go to the new crates/microsoft-fast-convert/src/syntax/webui_framework_prerelease.rs (shadow-DOM/f-ref/event logic, strip-with-warning), src/warning.rs (ConvertWarning/ConvertOutput), and src/wasm.rs's ConvertResult struct. syntax/webui.rs is untouched (restored to its original, pre-PR content). packages/fast-build/bin/fast.js has one updated call site that now destructures { output, warnings } for every target, and test/config.test.js has a new CLI test exercising --syntax=webui-framework-prerelease's warning output.

📑 Test Plan

• cargo test in crates/microsoft-fast-convert: all tests pass (metadata for all 3 targets, original webui-prerelease behavior, webui-framework-prerelease publisher/DOM mode, directive, event, strip-with-warning, robustness, and Avatar-fixture coverage).
• cargo clippy --all-targets and cargo fmt --check in crates/microsoft-fast-convert: clean.
• npm run build -w @microsoft/fast-build: wasm rebuilt successfully.
• npm run test:node -w @microsoft/fast-build: all 83 tests pass, including a CLI test asserting warnings are printed to stderr when webui-framework-prerelease strips f-slotted/f-children.
• Manual CLI smoke test converting a sample <f-template> with --syntax=webui-framework-prerelease confirmed shadowrootmode transfer, w-ref conversion, $e → e rewriting, and f-slotted stripping with a printed warning, end-to-end; a separate smoke test with --syntax=webui-prerelease confirmed output is unchanged from before this PR.
• npm run checkchange and npm run biome:check: pass.

✅ 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.

Agents

• I have linked to an existing issue in this project that this change addresses
• I have read the skills
• I have read the DESIGN.md file(s) in packages relevant to my changes
• I have updated the DESIGN.md file(s) in packages relevant to my changes

⏭ Next Steps

• The design doc also proposes an optional, typed WebUIConvertOptions (e.g. configurable event-object/unsupported-directive modes). This remains deferred as out of scope; a follow-up PR can add it if needed.
• crates/microsoft-fast-convert's own crate version was not bumped as part of this PR (unrelated to the beachball change file added for @microsoft/fast-build's wasm artifact); that's left to the maintainers' normal crates.io release process.