Implement instance methods

[b-temirov]

This PR adds class-method support for non-inheriting classes by lowering instance methods to standalone direct-call wasm functions with an explicit this parameter, and a prescan that predeclares constructors and methods so self recursion, mutual recursion, later-method calls, and constructor calls to same or later classes work by construction.

[LPTK]

Is this ready for review/merge?

[b-temirov]

Is this ready for review/merge?

Not quite. Inheritance coming soon on top. Or should I raise inheritance separately?

[LPTK]

Yes, better keep the PRs small and merge them regularly.

[Copilot]

Pull request overview

Adds WebAssembly backend support for instance/class methods on (non-inheriting) classes by lowering methods into standalone direct-call Wasm functions with an explicit receiver (this) parameter, plus a prescan that predeclares constructors/methods to support recursion and forward references.

Changes:

• Add prescan metadata registration for constructors and methods, and emit lowered method functions.
• Route constructor/method calls through direct call to the lowered functions (instead of indirect call_ref) where applicable.
• Add/adjust Wasm golden tests for methods and for constructor calls in matching output.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
hkmc2/shared/src/test/mlscript/wasm/Methods.mls	New golden tests covering getters, nullary/callable methods, recursion, forward references, chaining, mutation, and error cases.
hkmc2/shared/src/test/mlscript/wasm/Matching.mls	Update golden WAT output to reflect direct constructor calls (call) rather than call_ref.
hkmc2/shared/src/main/scala/hkmc2/codegen/wasm/text/WatBuilder.scala	Implement method/ctor metadata prescan, emit lowered method funcs, and add direct-call lowering for constructors/methods and related selection/assignment handling.
hkmc2/shared/src/main/scala/hkmc2/codegen/wasm/text/Ctx.scala	Add MethodInfo/ConstructorInfo metadata storage and accessors in the Wasm codegen context.

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[LPTK]

The logic seems quite complicated, for what this achieves. But let's try to merge this soon anyway, as soon as you have fixed what I point out.

[Derppening]

Just a few preliminary comments for now. I will need to look into this in more detail.

[b-temirov]

This PR has been superseded by #456.