Support bare return

[wtholliday]

Fixes the remaining half of #23.

Stacked on #28 — based on fix-return-type-check, since a bare return needs the return-type constraint that PR adds in order to be checked against the enclosing function. Please merge #28 first; the base will retarget to main automatically.

The problem

return with no value was a parse error. parse_stmt unconditionally parsed an operand, and Expr::Return held a non-optional ExprID, so there was no way to represent it:

do_work(done: bool) {
    if done {
        return
    }
    value = value + 1
}

do_work.lyte:6: Expected expression

The error pointed at the line after the return, and in a larger file the desynced parser cascaded into a run of bogus Expected declaration errors — which is what made this hard to diagnose from the reporter's side.

The fix

Expr::Return(Option<ExprID>). The parser treats return as bare when nothing is left in the statement — Endl, Rbrace, or end of input — and the checker types a bare return as void, so it unifies with the enclosing function's return type through the machinery from #28.

The issue asked for either support or a clearer diagnostic. This gives both: bare return works as an early exit, and using one in a value-returning function is now a clear type error rather than a parse failure.

❌ f.lyte:2:5: return type must match function return type: void vs i32
    return
    ^

Each backend emits its existing void-return path for the bare case: return_(&[]) in Cranelift, build_return(None) in LLVM, ReturnVoid in the stack VM, and a zero placeholder in r0 for the register VM (whose void epilogue ignores it).

Two bugs fixed along the way

• return g() where g is itself void fed a placeholder value to a void signature in the Cranelift and stack backends. Both now emit the void return. LLVM already special-cased this.
• In the stack backend, Expr::Return in void context fell through to the generic "translate then drop" path, emitting a Drop after the return terminator. Return never falls through, so there is nothing to drop.

Testing

New golden tests: checker/bare_return (the issue's example, plus unreachable code after a bare return), checker/bare_return_in_value_fn (the error case), loops/bare_return_in_loop (returns from the function, not just the loop). Plus a pretty_print unit test for the bare form.

cargo test --workspace passes: 352 lib tests and 313 golden tests across all four backend suites.

I also built with --features llvm and ran the bare-return cases through the LLVM backend by hand, since it isn't part of the default test run. All four backends — Cranelift JIT, stack VM, register VM, LLVM — produce identical output for bare returns inside loops, inside if/else, in lambdas, and with unreachable code following.

No new cargo fmt drift or clippy warnings relative to the base (both verified by diffing against a stashed working tree; the repo has pre-existing drift in other files).

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https://claude.ai/code/session_01BX9w4k8qecsH9LMgBP8hXc