fix(wasm): #1081 — invalid stack balance on Stmt::Return of void call

[proggeramlug]

Closes #1081.

Summary

• WASM validator was rejecting Mango's compiled .wasm with expected 1 elements on the stack for return, found 0. The trigger is an expression-bodied arrow whose body is a void call, e.g. v => console.log(v).
• Such an arrow lowers to a closure with Stmt::Return(Some(console.log(v))). The closure's WASM signature is [i64] -> [i64], but emit_expr for console.log/warn/error (and other emit_memcall_void paths) returns from the match arm without pushing a result. The subsequent Instruction::Return then violates the function signature.
• Mirror JS semantics (a void call evaluates to undefined) by pushing TAG_UNDEFINED when expr_has_value(e) is false in Stmt::Return(Some(e)). Same pattern Stmt::Expr already uses (Drop only when expr_has_value).

Files

• crates/perry-codegen-wasm/src/emit.rs — push TAG_UNDEFINED after emit_expr in Stmt::Return(Some(e)) when the expression is void.
• tests/wasm/22_return_void_expr.{ts,expected} — regression test covering g().then(v => console.log(v)) and const log = (v: number) => console.log(v).

Test plan

• WebAssembly.compile accepts the produced module (was failing without the fix).
• All 6 candidate repros from the issue triage compile (C1–C6 plus the c2c/c2d/c2e variants that isolate the void-call closure body).
• tests/wasm/run_wasm_tests.sh: 15 passed / 7 failed; the 7 pre-existing failures match main and the new 22_return_void_expr passes.
• cargo fmt --all; cargo build --release -p perry-codegen-wasm -p perry-runtime -p perry-stdlib -p perry.

No version bump or changelog change — maintainer will fold those in at merge time.