chore(fast-html): use @microsoft/fast-build to build when fixture with lowercase attrs

[janechu]

Pull Request

📖 Description

Adds the when test fixture to the @microsoft/fast-html package's build-fixtures.js script so it is generated by @microsoft/fast-build SSR alongside the existing attribute, binding, event, ref, and slotted fixtures.

This PR also includes a fix to @microsoft/fast-build where entry-level root custom elements now merge the full root state with their own per-element attribute-derived state when rendering their shadow template. Previously, entry elements received only the full root state, which broke elements whose template expressions depend on per-element HTML attributes (e.g. planet="earth", vara="3", boolean show). Attribute-derived values take precedence over root state for overlapping keys.

The nested-when template expressions are updated to use camelCase identifiers (showProgress, enableContinue) matching both the state.json keys and the element's @observable property names, ensuring correct server-side resolution and correct FAST hydration binding.

📑 Test Plan

All existing Rust unit tests pass (cargo test in crates/microsoft-fast-build). Fixture output verified by running npm run build:fixtures — only the when fixture changed (no regressions in attribute, binding, event, ref, or slotted fixtures). The when/when.spec.ts Playwright tests cover the full lifecycle: initial SSR state, FAST hydration, and reactive property changes.

✅ 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]

Updated to pass state attributes to the nested-when element. Now builds successfully with the renderer fix in #7396 (kebab-to-camelCase attribute mapping).

[Copilot]

Pull request overview

Updates the @microsoft/fast-html fixture build pipeline to include the when fixture in SSR generation via @microsoft/fast-build, and adjusts microsoft-fast-build so entry-level (root) custom elements can render shadow templates using both root state and per-element attribute-derived state.

Changes:

• Add the when fixture to packages/fast-html/scripts/build-fixtures.js and introduce when fixture inputs (entry.html, templates.html, state.json) + regenerated index.html.
• Change microsoft-fast-build root custom element rendering to merge root JSON state with attribute-derived state for shadow template evaluation.
• Update microsoft-fast-build documentation (README + DESIGN) to describe merged-state behavior for entry rendering.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
packages/fast-html/test/fixtures/when/templates.html	New <f-template> declarations for the when fixture used by fixture injection/hydration.
packages/fast-html/test/fixtures/when/state.json	New fixture state used for SSR generation.
packages/fast-html/test/fixtures/when/main.ts	Defines the fixture custom elements used by the templates/tests.
packages/fast-html/test/fixtures/when/index.html	Regenerated SSR output + injected templates for the fixture page.
packages/fast-html/test/fixtures/when/entry.html	New entry HTML input used by fast-build to generate the fixture SSR output.
packages/fast-html/scripts/build-fixtures.js	Adds when to the list of fixtures rendered via @microsoft/fast-build.
crates/microsoft-fast-build/src/directive.rs	Implements merged root+attribute state for entry-level custom elements.
crates/microsoft-fast-build/README.md	Updates docs to explain merged-state behavior and root-element attribute handling.
crates/microsoft-fast-build/DESIGN.md	Updates design notes for is_entry semantics to match merged-state behavior.
change/@microsoft-fast-html-when-00615379-81a9-4dd0-86af-bd600a8ab155.json	Records the @microsoft/fast-html change entry.

Comments suppressed due to low confidence (1)

crates/microsoft-fast-build/src/directive.rs:285

• build_attr_state currently skips all attributes that start with ?. That avoids mis-resolving expressions like ?disabled="{{!enableContinue}}", but it also means ?attr boolean bindings on a custom element tag can never influence the element’s shadow-template rendering state (even though entry-tag rendering does evaluate ?attr). Consider evaluating ?attr="{{expr}}" with evaluate() and inserting a boolean into the child state (under the attr name without ?), so SSR state and rendered HTML stay consistent.

        let attrs = parse_element_attributes(open_tag_content);
        let mut state_map = base.unwrap_or_default();
        for (attr_name, value) in &attrs {
            // Skip @event handlers — they are client-side only and have no meaning in SSR state.
            // Also skip f-ref, f-slotted, f-children, and ?boolean-binding directives — all are
            // resolved entirely by the FAST client runtime.
            if attr_name.starts_with('@')
                || attr_name.starts_with('?')
                || attr_name.eq_ignore_ascii_case("f-ref")
                || attr_name.eq_ignore_ascii_case("f-slotted")
                || attr_name.eq_ignore_ascii_case("f-children")
            {
                continue;
            }