ERR/E2.1a: value-carrying failable producer (return value + raise → tuple ABI)

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.

tests/expected/228-value-failable.exit

@@ -0,0 +1 @@
+60

tests/expected/228-value-failable.txt

@@ -0,0 +1 @@
+value-failable result: 60