feat: propagate shadowroot attributes

[janechu]

Pull Request

📖 Description

• Propagates shadowroot* attributes from f-template onto rendered declarative shadow DOM <template> elements.
• Preserves the existing default shadowrootmode="open" and shadowroot="open" behavior when no mode is provided.
• Updates FAST build configuration/docs and adds Rust/Node coverage for explicit, boolean, escaped, and extra shadowroot attributes.

👩‍💻 Reviewer Notes

Please focus review on the shadowroot attribute normalization/escaping path shared between the Rust renderer and the JS CLI configuration bridge.

📑 Test Plan

Passed:

• git diff --check
• cargo test --manifest-path crates/microsoft-fast-build/Cargo.toml
• npm run build -w @microsoft/fast-build
• npm run test:node -w @microsoft/fast-build
• npm run build
• npm run biome:check
• npm run checkchange

✅ 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.

[janechu]

The parsing/escaping path is solid and the test coverage for the common cases is good. A few concerns left as inline comments. The most notable is an asymmetric shadowrootmode ↔ shadowroot mirroring: when a user specifies only shadowroot="closed" (no shadowrootmode), the output is <template shadowrootmode="open" shadowroot="closed">, which gives modern browsers "open" and legacy ones "closed". I verified this by running the new logic against that input. Other comments cover a boolean-vs-empty-value distinction, a public Rust API gap, missing test coverage for the asymmetric case, and a small JSON parsing nit.

[janechu]

Re-review of e674d99 — all six previous comments are addressed cleanly:

1. Asymmetric mirror ✅ — new explicit_shadowroot_attr_value helper plus the or_else/or chain mirrors shadowroot → shadowrootmode as well, so <f-template shadowroot="closed"> now yields shadowrootmode="closed" shadowroot="closed". New test test_f_template_legacy_shadowroot_value_mirrors_to_mode + fixture shadowroot-legacy-closed.html pin it down.
2. Boolean vs empty-string ✅ — explicit_shadowroot_attr_value filters empty strings out, so shadowrootmode="" is now treated identically to the boolean form. Four new fixtures + tests cover the empty cases (mode-only, shadowroot-only, mode-empty + shadowroot="closed", mode="closed" + shadowroot-empty).
3. Public Rust API gap ✅ — added add_template_with_shadowroot_attrs plus rustdoc on from_templates and add_template pointing users there. Unit test test_add_template_with_shadowroot_attrs_preserves_attrs added.
4. Test coverage gap ✅ — covered by item #1.
5. JSON null shadowrootAttributes ✅ — Some(json::JsonValue::Null) | None => Vec::new() arm added; wasm-test added (note: gated to target_arch = "wasm32", which matches the rest of wasm.rs).
6. Order-coupled JS assertion ✅ — refactored to extract the <template …> tag once and assert each attribute's presence individually.

Verified with cargo test (65 unit + 60 integration tests pass) and npm run test:node -w @microsoft/fast-build (30/30 pass). Also re-ran the truth table for build_shadowroot_template_attrs — explicit conflicts like shadowrootmode="open" shadowroot="closed" are still preserved verbatim, which is the right call.

One small follow-up left as an inline comment: the user-facing docs still only describe the one-way mode → shadowroot mirror.

[janechu]

Re-review of 0307207 — doc lag is fully addressed across all four files (crates/microsoft-fast-build/{DESIGN,README}.md, packages/fast-build/{DESIGN,README}.md). The new wording is consistent across locations and accurately describes the implemented behavior:

• Neither attribute non-empty → defaults to shadowrootmode="open" shadowroot="open"
• Exactly one has a non-empty value → mirrored to the other
• Both have explicit non-empty values → preserved as authored

Cross-checked against the truth table of build_shadowroot_template_attrs/explicit_shadowroot_attr_value — all four cases match.

No further blocking issues.