fix(web-sys): authoritative expansion model for stable/unstable WebIDL overrides

[guybedford]

Summary

• Fixes incorrect #[cfg(web_sys_unstable_apis)] gating on stable method signatures that share a WebIDL operation with unstable overloads
• Replaces the old JS-name-based override detection with an authoritative expansion model where stable and unstable signature sets are built independently and compared
• Adds typed generics (Promise<T>, Array<T>, Function<fn(...)>, etc.) to all unstable API methods
• Adds missing Image Capture spec types: PhotoCapabilities, PhotoSettings, MediaSettingsRange, Point2D, RedEyeReduction, FillLightMode, MeteringMode
• Fixes forward-reference problems in first pass by pre-registering interface/dictionary names
• Fixes per-interface maplike/setlike callback name collisions

Problem

When a WebIDL operation like read(optional ClipboardUnsanitizedFormats formats = {}) expands, the 0-arg read() doesn't use any unstable types, so it's a valid stable API. But the old code saw that unstable signatures existed for the same JS name (read) and incorrectly gated all signatures — including the stable 0-arg version — with #[cfg(web_sys_unstable_apis)].

This affected Clipboard.read(), WebGL texImage2D/texSubImage2D/texImage3D/texSubImage3D (VideoFrame overloads), Performance.measure(), Window/WorkerGlobalScope timer methods, and others.

Solution

The new authoritative expansion model treats stable and unstable WebIDL expansions as independent, authoritative sets:

1. Classify signatures as stable or unstable based on whether they come from unstable IDL definitions or use unstable types
2. Detect siblings — signatures from the same WebIDL definition as an unstable signature are included in the unstable set even if they're individually stable (e.g., read() is a sibling of read(options))
3. Build each set independently — naming disambiguation happens within each set
4. Compare and gate:
   • Same signature in both sets → merge (no cfg gate, available always)
   • Only in stable set → #[cfg(not(web_sys_unstable_apis))]
   • Only in unstable set → #[cfg(web_sys_unstable_apis)]

Additional Fixes

• Typed generics for unstable APIs: All unstable methods now use typed generics (Promise<T>, Array<T>, Function<fn(...)>) regardless of the generics compatibility mode for stable APIs
• Image Capture types: Added PhotoCapabilities, PhotoSettings, MediaSettingsRange, Point2D, RedEyeReduction, FillLightMode, MeteringMode from the W3C Image Capture spec — these were referenced but never defined, causing methods to be silently skipped
• Phase 0 pre-registration: Interface and dictionary names are pre-registered before processing members, fixing forward-reference failures (e.g., iterable over a type defined later in the same file)
• Per-interface callback names: Maplike/setlike forEach callback type names are now per-interface ({Interface}MapLikeForEachCallback) instead of global singletons that overwrote each other
• Error propagation: ArrayTuple type conversion now propagates errors instead of silently falling back to JsValue
• Signature derives Eq/Hash: Replaced *const Signature pointer identity with proper structural equality via HashSet<&Signature>

Key Files Changed

• crates/webidl/src/util.rs — authoritative expansion model in create_imports()
• crates/webidl/src/generator.rs — same_signature(), generics_compat logic for unstable APIs
• crates/webidl/src/lib.rs — Options with Cell<bool>, generics_compat in dictionary/const/namespace/iterable code
• crates/webidl/src/first_pass.rs — Phase 0 registration, per-interface callback names, Eq/Hash on Signature
• crates/webidl/src/wbg_type.rs — error propagation in ArrayTuple
• crates/web-sys/webidls/unstable/ImageCapture.webidl — added missing types from W3C spec
• crates/web-sys/src/features/ — regenerated bindings

Testing

• just test-web-idl-tests — stable mode tests pass
• just test-web-idl-tests-unstable — unstable mode tests pass
• just test-web-idl-tests-next — next mode tests pass
• just generate-web-sys — regenerated bindings verified