fix: make handle<T> surface consistently across generators

[owenthcarey]

Summary

handle<T> (a typed opaque handle whose tag is a struct T) is rendered differently by each generator, and two of the renderings do not compile. The IDL reference says it is "still a uint64_t" at the C ABI, but the emitted header declares it as an opaque struct pointer (weaveffi_<module>_<T>*), and the language wrappers disagree about what a consumer receives.

Observed on main after #44, using the kitchen_sink and edge_cases snapshot fixtures:

Generator	handle<Token> surfaces as	Status
C header	weaveffi_kitchen_Token* (opaque pointer)	compiles; contradicts the docs
Dart	Token._(result) / token._handle on the Token record class	does not compile: Token is a plain data class with no _ constructor or _handle getter
Node .d.ts	Token (the record interface itself)	compiles but wrong: a pointer is typed as the decoded record
Swift	UInt64	compiles
Python	int (c_void_p)	compiles
Wasm .d.ts	number	compiles

The Dart failure predates #44 and was never caught because no sample uses handle<T> and nothing compiled the generated Dart before the edge_cases audit.

Reproduction

1. weaveffi generate crates/weaveffi-cli/tests/fixtures/edge_cases.yml -o out
2. cd out/dart && dart analyze
3. See The getter '_handle' isn't defined for the type 'Leaf' and The method '_' isn't defined for the type 'Leaf' at the typedHandleParam wrapper. The same Token._(result) shape appears in the kitchen_sink Dart output.

Expected

One documented answer to "what does a consumer hold for handle<T>?" applied uniformly. Two reasonable designs:

• Opaque wrapper class per tag. Each language gets a tiny TokenHandle-style class wrapping the pointer, giving the type safety the docs promise. This is what Dart appears to have been reaching for.
• Plain integer everywhere. Match Swift and Python, and fix the docs' uint64_t claim to agree with the header (or change the header to uint64_t).

Either way, Dart must stop calling members that do not exist, Node must stop typing the handle as the record, and docs/src/reference/idl.md ("Typed handles") must match the header.

Environment

• WeaveFFI version: main after feat!: resolve types to Ty, and move package config to weaveffi.toml #44
• Target generator(s): C / Dart / Node / Swift / Python / Wasm (docs)

Additional context

Found during the coverage audit in #44. The edge_cases fixture keeps typed_handle_param (handle<Leaf> as both a parameter and a return) so the snapshot diff will show the fix landing in every generator.