The producer side of the error-channel tuple ABI for value-carrying `-> (T, !)`
functions. A failable that returns a value OR an error now lowers correctly;
the result is consumed via destructure (`v, err := f()`). Single-value
`-> (T, !)`; multi-value `-> (T1, T2, !)` and the value-carrying try/catch
consumers (E2.1b) follow.
- lowerReturn: a value-carrying failable's `return v;` assembles the success
tuple `{v, 0}` (compiler appends the no-error slot) via lowerFailableSuccessReturn
(tuple_init). Forwarding a full failable tuple (`return other_failable()` /
explicit `return (v, e)`) returns as-is. Multi-value returns bail loudly (E2).
- lowerRaise: the value-carrying branch (previously a loud bail) now builds
`{undef value slots..., tag}` (constUndef per value slot + the error tag) and
returns it — any arity.
- helpers: buildFailableTuple (tuple_init from value refs + tag) + emitTupleRet
(return honoring inline-comptime targets).
Value-carrying `try` / `catch` still bail (E2.1b). Tests:
examples/228-value-failable.sx (return value + both raises, consumed by
destructure; exit 60). Gates: zig build, zig build test, 266/266 examples.
