feat: expose dom option on the Node binding's build() API

[janechu]

What

Exposes the dom build option on @microsoft/webui's napi-rs Node binding so JavaScript callers can opt into light DOM rendering.

Why

The CLI already supports webui build --dom light and the Rust API already supports BuildOptions { dom: DomStrategy::Light, .. }. The Node binding's runtime build() function, however, hardcoded DomStrategy::Shadow and gave JS callers no way to select light DOM.

Light DOM rendering is the supported path for component output that needs to compose correctly when injected after page load - for example, into a streaming response body whose later HTML lands via innerHTML and therefore cannot rely on DSD auto-attachment. The fast-v3 plugin emits only HTML comment markers (<!--fe:b-->, <!--fe:/b-->, <!--fe:r-->, <!--fe:/r-->) and data-fe="N" attributes, all of which work identically in light DOM, so --dom light --plugin fast-v3 is a fully supported combination.

What changed

• crates/webui-node/src/lib.rs
  • JsBuildOptions gains pub dom: Option<String>.
  • build() parses the field via the existing webui::DomStrategy::FromStr impl (accepts "shadow" and "light") and falls back to webui::DomStrategy::default() (Shadow) when omitted.
  • Replaces the hardcoded dom: webui::DomStrategy::Shadow in BuildOptions.
  • Adds three tests:
    • test_build_with_light_dom_omits_shadow_root_template
    • test_build_with_shadow_dom_emits_shadow_root_template
    • test_build_invalid_dom
• docs/guide/integrations/node.md
  • Adds a dom row to the BuildOptions reference table, matching the existing CLI documentation.

Non-breaking

• New field is Option<String>. Omitted by every existing caller -> falls back to DomStrategy::default() (Shadow) -> byte-for-byte identical output.
• No protocol changes (DomStrategy is already in the proto schema).
• No CLI or Rust API changes.

Validation

• cargo test -p microsoft-webui-node --lib: 24 passed.
• cargo xtask check: license-headers, fmt, clippy all green. (The proto-drift step requires protoc which isn't available in my local env; this PR touches no .proto files.)