feat: preserve shadowroot* attrs on component template wrapper

[janechu]

Summary

When DomStrategy::Shadow is active, the WebUI parser now preserves any shadowroot-prefixed attributes the user wrote on their wrapping <template> and applies these rules:

• Mode — shadowrootmode value (or, if absent, a bare legacy shadowroot= attribute) is captured. Both attributes are then always emitted on the wrapper as shadowrootmode="<mode>" shadowroot="<mode>" (the legacy shadowroot attribute is emitted as a paired alias for older user agents / tooling that look for either form). Defaults to "open" when neither is supplied.
• Other shadow-root attrs — shadowrootclonable, shadowrootdelegatesfocus, shadowrootserializable, etc. are preserved verbatim on the emitted <template>.
• shadowrootadoptedstylesheets — when the user supplies a value AND the CSS Module strategy would also generate one, the values are merged into a single space-separated list (user-supplied specifiers first in their original order; parser-generated specifiers appended only if not already present, deduplicated).
• Runtime / non-shadowroot user attrs on <template> — @event, :prop, ?bool, and any other non-shadowroot* attribute on the user's <template> is stripped from the emitted wrapper (existing behaviour).

Under the FAST v2 / v3 parser plugins those shadowroot* attributes are placed on the outer <f-template> element rather than the inner <template>. <f-template> is the framework-owned transport element that the FAST runtime consumes; the inner <template> always has all shadowroot* attributes stripped so the browser does not auto-activate it as a declarative shadow root inside <f-template>. The same shadowrootadoptedstylesheets merge rule applies on <f-template>.

Bug fix included

Also fixes a pre-existing bug in generate_f_template (fast-v2 / fast-v3) that used find('>') to locate the end of the outer <template> opening tag — this broke on attribute values containing > (e.g. data-note="a > b"). The new implementation uses the existing find_tag_close helper which respects quoted attribute values, with a regression test in both plugins.

Backward compatibility

Light-DOM mode is unaffected: the <template> wrapper continues to be stripped entirely. The default shadow-DOM behaviour (no user shadowroot* attrs supplied) emits <template shadowrootmode="open" shadowroot="open">, which is backward-compatible with existing apps that rely on shadowrootmode="open".

Tests

• 8 new tests in the parser core (crates/webui-parser/src/lib.rs) covering: default paired emission, user-supplied shadowrootmode="closed", bare legacy shadowroot= promoted to mode, shadowrootclonable/shadowrootdelegatesfocus preservation, adopted-stylesheets merge with user value, dedup against module specifier, runtime attrs stripped from wrapper, and the light-DOM regression.
• 7 new tests in each of fast_v2.rs and fast_v3.rs (14 total) covering: shadowrootmode moved to <f-template>, user closed mode preserved, other shadow-root attrs on <f-template>, no shadowroot* on <f-template> when user did not supply any, module CSS strategy puts shadowrootadoptedstylesheets on <f-template>, merged with user value, and the > in quoted attr regression test. Plus updated existing tests to assert the new placement and a coverage test for the renamed strip_shadowroot_attrs helper.
• All 386 parser tests pass.
• Full cargo xtask check passes (license-headers, fmt, clippy, deny, test, build, build (wasm), build (examples), bench, docs).

Documentation

• DESIGN.md — new "Shadow DOM <template> wrapper" subsection documenting the preservation, paired-alias, merge, and FAST-plugin placement rules.
• docs/guide/concepts/components/index.md — updated component-author guidance.
• docs/guide/cli/index.md — updated DOM strategy table.
• docs/ai.md — updated AI-targeted guidance.
• packages/webui-framework/README.md and RENDERING.md — example output reflects paired shadowrootmode/shadowroot.
• Doc-comments on process_component_template, generate_f_template, convert_btr_to_fast, and strip_shadowroot_attrs describe the new behaviour.

[mohamedmansour]

I don't want us to capture each individual user supplied attributes, we should preserve ALL the attributes if the webui component supplied them.