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.

examples/235-multi-value-failable.sx

@@ -0,0 +1,64 @@
+// Multi-value value-carrying failables (ERR — the multi-value error-channel
+// ABI). A `-> (T1, T2, !E)` function returns EITHER a value-tuple OR an error:
+// `return (a, b)` yields the success tuple `{a, b, 0}` (the compiler appends the
+// no-error slot) and `raise error.X` yields `{undef, undef, tag}`. Every consumer
+// generalizes from the single-value shape: a destructure binds every slot
+// INCLUDING the error (dropping it is the spec'd discard error — bind it and
+// inspect); `try` binds the value-tuple on success and propagates `{undef..., tag}`
+// on failure; `catch` / `or` absorb the failure and merge the value-tuple or the
+// handler/terminator value. Single-value `-> (T, !E)` is examples/228-231.
+
+#import "modules/std.sx";
+
+E :: error { Bad, Empty }
+
+parse :: (n: s32) -> (s32, s32, !E) {
+    if n < 0  { raise error.Bad; }
+    if n == 0 { raise error.Empty; }
+    return (n * 2, n + 1);            // success → {n*2, n+1, 0}
+}
+
+// Multi-value `try` in a multi-value caller — propagates {undef, undef, tag}.
+inc :: (n: s32) -> (s32, s32, !E) {
+    v, b := try parse(n);
+    return (v + 1, b + 1);
+}
+
+// Multi-value `catch`, bare-expression tuple fallback (absorbs the failure).
+safe :: (n: s32) -> s32 {
+    v, b := parse(n) catch e (40, 50);
+    return v + b;
+}
+
+// Multi-value `or (tuple)` value-terminator (absorbs the failure).
+ortest :: (n: s32) -> s32 {
+    v, b := parse(n) or (7, 8);
+    return v + b;
+}
+
+main :: () -> s32 {
+    r : s32 = 0;
+
+    // Destructure binds EVERY slot including the error tag (e1 / e2 / e3) —
+    // the error is treated, never dropped.
+    v1, b1, e1 := parse(5);          // success → (10, 6, no-error)
+    if e1 == error.Bad { r = r + 1000; }  // false
+    r = r + v1 + b1;                 // +16
+
+    v2, b2, e2 := parse(-1);         // Bad → {undef, undef, Bad}
+    if e2 == error.Bad { r = r + 4; }   // +4
+
+    a, c, ea := inc(5);              // parse(5)=(10,6) → (11, 7, no-error)
+    if ea == error.Bad { r = r + 2000; }  // false
+    r = r + a + c;                   // +18
+
+    a2, c2, e3 := inc(-1);           // try parse(-1)=Bad → propagate {undef, undef, Bad}
+    if e3 == error.Bad { r = r + 5; }   // +5
+
+    r = r + safe(5);                 // (10, 6) → 16
+    r = r + safe(-1);                // Bad → catch → (40, 50) → 90
+    r = r + ortest(0);               // Empty → or → (7, 8) → 15
+
+    print("multi-value result: {}\n", r);  // 16+4+18+5+16+90+15 = 164
+    return r;
+}