[Jane Chu (janechu)]

Pull Request

📖 Description

Adds structured build-time diagnostics so integrations can report authored HTML locations without parsing error messages or searching for repeated expressions.

WASM render, conversion, composition, and metadata-validation failures now throw actual Error objects with stable codes and optional UTF-16 locations/template names. Native Rust accessors and generated TypeScript interfaces expose the same details. Existing human-readable messages, successful output, warnings, permissive metadata parsing, and missing-state behavior are preserved.

🎫 Issues

Closes #7750.

👩‍💻 Reviewer Notes

Entry/conversion locations refer to the original HTML, including after preprocessing and recursive extraction. Separately supplied component failures use their supplied content and templateName, never invented entry-file coordinates. Non-HTML failures omit source coordinates.

Intentional pre-1.0 API changes: JavaScript catch values change from strings to Error objects; native context fields use ErrorContext, and source-annotated validation errors may use Located. The migration guide covers .message, .kind(), and source selection.

There are no fast-element client-runtime changes. Diagnostic contexts/source remapping are created only on failure; locator definitions retain an additional original string only when code-sample escaping changed their content.

📑 Test Plan

Regressions were confirmed failing before implementation. Coverage includes actual Error instances/stacks, codes, exact UTF-16 coordinates/highlight lengths, CRLF/lone-CR/LF, duplicate markup, nested when/repeat/light DOM, named nested definitions, code-sample escaping, all converter targets/publisher rewriting, validation wrappers, absent non-HTML coordinates, and generated declarations.

Command	Result
cargo test --manifest-path crates/microsoft-fast-build/Cargo.toml --quiet	Passed: complete native renderer suite
cargo test --manifest-path crates/microsoft-fast-convert/Cargo.toml --quiet	Passed: complete native converter suite
cargo test --manifest-path crates/microsoft-fast-convert/Cargo.toml --test errors --quiet	Passed: 24 error tests after final equivalent Unicode-escape cleanup
npm run build -w @microsoft/fast-build	Passed: rebuilt both WASM modules
node --test packages/fast-build/test/plugin-integrations.test.js	Passed: all 14 integration/type tests
npm run build -- --no-cache	Passed: all 9 workspace tasks
npm run test -- --no-cache	Passed: all 14 workspace tasks, including browser suites
npm run biome:check	Passed
npm run biome:check -- --staged	Passed: changed JavaScript checked before commit
npm run format:check	Passed; no additional files selected by its changed-file filter
npm run checkchange	Passed after committing the generated change file
npm run prebuild -w sites/website	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.

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

None required for this issue.