Port JS scoped-blocks and remove definedVars

[b-temirov]

This PR ports JSBuilder scoped-block local allocation semantics to wasm backend. It removes definedVars local collection and switches to scope-driven allocation.

[Copilot]

Pull request overview

Ports JSBuilder-style scoped-block local allocation to the Wasm text backend by removing reliance on definedVars and adding new coverage for scoped locals behavior in the Wasm tests.

Changes:

• Add ScopedLocals.mls Wasm tests covering local allocation across top-level lets, chaining, if/else branches, and match arms.
• Update WatBuilder to (a) handle Assign(State.noSymbol, ...) by evaluating and dropping results, and (b) switch block-local allocation to scope-/Scoped-driven allocation via withLocalDelta + nonNestedScoped.

Reviewed changes

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

File	Description
hkmc2/shared/src/test/mlscript/wasm/ScopedLocals.mls	Adds new Wasm test cases validating scoped-local allocation semantics (and some WAT snapshots).
hkmc2/shared/src/main/scala/hkmc2/codegen/wasm/text/WatBuilder.scala	Implements scope-driven local allocation, removes definedVars dependency, and adds a noSymbol assignment handling path.

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[LPTK]

Following the change proposed by Copilot, I realize that there's a bunch of junk in the WatBuilder that was mindlessly copied from JSBuilder, such as def body now being unused. As a more serious example, it doesn't make any sense to use things like scope.nestRebindThis, as this deals with details of how JavaScript deals with the this keyword (which doesn't even exist in Wasm) in nested definitions (which aren't even possible in Wasm).

Please make a pass to remove and simplify this unnecessary logic.

[b-temirov]

1. Removed dead code:

• Deleted unused body helper.
• Removed unused name parameter from setupFunction.

2. Removed JS-specific this rebinding flow:

• Replaced scope.nestRebindThis(...) with scope.nest in Define handling.
• Removed thisProxy / thisProxyDefined branch and error path.
• Replaced scope.findThis_! usages with explicit scope/local lookup.

3. Added explicit constructor self binding:

• Introduced bindCtorThis(...) to bind constructor isym directly to local name "this" in wasm scope.

[LPTK]

3. Added explicit constructor self binding:

There is no test to accompany this change. There should be at least one WASM test that uses this.

[b-temirov]

Added two new test cases to Basics.mls.

[LPTK]

Thanks, LGTM now. Except for this one small question.