fix(wasm): resolve E0308 — bet-wasm + whole Rust workspace build green (#64)

## Fix `bet-wasm` E0308 — whole Rust workspace now builds green

`bet-wasm` was the one Rust crate that didn't compile (long flagged
"paused"). The default-return scaffold's `match ret_ty` had
**incompatible arm types**: the `I32` arm was a statement block (`()`),
while `I64`/`F32`/`F64` were bare `func.instruction(...)` expressions
(which return `&mut Function`). The match is purely side-effecting (it
emits a default-return instruction; its value is unused), so all arms
are now `()` statement blocks.

### Verified (locally, via cargo)
- `cargo test -p bet-wasm` → **8 passed**
- `cargo build --workspace` → **exit 0** — every Rust crate now compiles
(bet-syntax, bet-parse, bet-core, bet-check, bet-eval, bet-codegen,
bet-wasm).

README status refreshed: WASM backend **paused → builds (scaffolding)**
— it compiles and emits default-return stubs; real AST→WASM lowering is
still pending (that's separate work, not claimed here).

### Scope
This is the **build-unblock only**. The other half of the "finish the
Rust pipeline" item — raising `bet-eval`/`bet-codegen` toward parity
with the authoritative Racket interpreter — remains follow-on work.

https://claude.ai/code/session_01QGi8GND5yNWgDyfReVEPYs

---
_Generated by [Claude
Code](https://claude.ai/code/session_01QGi8GND5yNWgDyfReVEPYs)_

Author: hyperpolymath

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
 |===
 
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