fix(wasm): resolve E0308 so bet-wasm (and the whole Rust workspace) builds

The default-return scaffold's `match ret_ty` had inconsistent arm types: the
I32 arm was a statement block (`()`), while I64/F32/F64 were bare
`func.instruction(...)` expressions (`&mut Function`). The match is purely
side-effecting (emits a default-return instruction; its value is unused), so
all arms are now `()` statement blocks.

Verified: `cargo test -p bet-wasm` → 8 passed; `cargo build --workspace` → exit
0 (every Rust crate now compiles). README status refreshed (wasm: paused →
builds; still scaffolding pending AST lowering).

Scope: this is the build-unblock only. Raising bet-eval/bet-codegen toward
parity with the authoritative Racket interpreter remains follow-on work.

https://claude.ai/code/session_01QGi8GND5yNWgDyfReVEPYs

Author: claude

README.adoc

@@ -96,7 +96,7 @@ for the mechanised fragment.
 * `compiler/bet-core/` — core type definitions (incl. `Echo T`, `EchoR T`)
 * `compiler/bet-check/` — Hindley-Milner type checker + unifier (27 tests pass)
 * `compiler/bet-parse/` — LALRPOP-based surface parser
-* `compiler/bet-wasm/` — WASM backend (**paused**, pre-existing build issue)
+* `compiler/bet-wasm/` — WASM backend (builds; scaffolding — emits default-return stubs pending AST lowering)
 
 Rust is the compiler tooling layer. Core semantics remain in Racket.
 
@@ -453,7 +453,7 @@ BetLang is built on three principles:
 | VS Code extension | 🟡 In progress | AffineScript source (replaces ReScript)
 | Web Playground (ui/) | ✅ Available | Quantum-inspired features
 | FFI (Zig) | 🟡 Experimental | playground/ffi/zig/
-| WASM backend | ⏸️ Paused | Pre-existing build issue in bet-wasm
+| WASM backend | 🟡 Builds (scaffolding) | E0308 fixed; emits default-return stubs pending AST lowering
 |===
 
 ---

compiler/bet-wasm/src/lib.rs

@@ -552,9 +552,15 @@ impl WasmBackend {
                             func.instruction(&Instruction::I32Const(0));
                         }
                     }
-                    WasmType::I64 => func.instruction(&Instruction::I64Const(0)),
-                    WasmType::F32 => func.instruction(&Instruction::F32Const(0.0)),
-                    WasmType::F64 => func.instruction(&Instruction::F64Const(0.0)),
+                    WasmType::I64 => {
+                        func.instruction(&Instruction::I64Const(0));
+                    }
+                    WasmType::F32 => {
+                        func.instruction(&Instruction::F32Const(0.0));
+                    }
+                    WasmType::F64 => {
+                        func.instruction(&Instruction::F64Const(0.0));
+                    }
                 }
             }