ERR/E2: multi-value failables -> (T1, ..., !)

Generalize the single-value `-> (T, !)` error-channel ABI to any value
arity. Retire the five `fields.len == 2` bails (lowerFailableSuccessReturn,
lowerTry, lowerCatch, lowerFailableOr, and the inferExprType try/catch/or
arms); lowerRaise + emitErrorReturn already looped over N value slots.

New helpers centralize "value-part = every slot but the last (error) one":
failableSuccessType (lone value type, or a value-tuple), extractSuccessValue,
extractErrorSlot.

Fix one latent bug the feature surfaced: coerceToType had no tuple->tuple
arm, so a value-tuple flowing into a differently-typed success slot (e.g.
(s64,s64) catch body into (s32,s32)) fell through unchanged. Add element-wise
coercion. No lowerTupleLiteral change is needed: a `return (a, b)` literal
against a 3-field failable target already gets target_fields=null via the
arity mismatch, so it types as a plain value-tuple that
lowerFailableSuccessReturn consumes.

examples/235-multi-value-failable.sx exercises producer return/raise,
destructure (binding every slot incl. the error tag), multi-value try
(success + propagation), catch (bare-expr tuple body), and or-tuple
terminator. Match-body tuple arms are left out: `(` after `case PAT:` is
parsed as a payload capture (a pre-existing, multi-value-unrelated parser
bug). Gates: zig build, zig build test, 273/273 examples.

tests/expected/235-multi-value-failable.exit

@@ -0,0 +1 @@
+164

tests/expected/235-multi-value-failable.txt

@@ -0,0 +1 @@
+multi-value result: 164