[adamcavendish]

Summary

Migrate TypeScript Fetch API interfaces from formatted signature fragments to structural TypeSpec, FieldSpec, and CallableParam emission.

Problem

API interface methods were assembled as raw source strings because the emitter treated named callable parameters as unsupported. That bypassed structural type composition and made the interface surface a remaining exception to the generator's sigil-stitch pipeline.

Behavior

Surface	Result
API interface properties	Required request parameters and optional initOverrides are represented as named callable parameters.
Type and value imports	Callable parameter and return types remain structural through sigil-stitch lowering and import collection.
Generated clients	Request signatures, response types, class implementation, and runtime behavior remain unchanged.
Edge cases	No-input, cookie-only, optional query/body, path parameters, named arrays, and nullable array items are covered.
Output changes	Existing TypeScript golden differences are limited to whitespace and continuation layout within API interface declarations.

Compatibility and boundaries

• This PR is intentionally based on PR #160; review or merge it after that base change.
• The local sigil-stitch checkout is used only for validation. No machine-local Cargo configuration, Cargo.lock change, dependency patch, version bump, or release is included.
• The published sigil-stitch 0.7.3 composition prerequisite documented in PR refactor: structure TypeScript response dispatch #160 remains unresolved; local fixed-checkout validation must not be treated as registry-only validation.