fix(issue-0057): all-diverging match arms no longer fail LLVM verification

A match (`if subject == { case ... }`) whose arms all diverge (each
`return`s / `raise`s) failed LLVM verification with a `void` phi plus
"Terminator found in the middle of a basic block". Two causes in lowerMatch:

- The value-arm path did `lowerBlockValue(arm.body) orelse constInt(0, …)`,
  emitting the fallback `const` into a block the body had ALREADY terminated
  (a diverging arm), so `currentBlockHasTerminator()` then saw the const (not
  the `ret`) and emitted a `br merge` after the terminator. Fix: materialize
  the fallback value + branch only when the block hasn't terminated.
- A fully-diverging match infers `result_type == .noreturn` yet still built a
  value-merge phi. Fix: `has_value_merge` excludes `.noreturn`, so such a
  match builds no phi; its arms terminate and the merge block is unreachable.

Also: inferMatchResultType now skips `.noreturn` arms (a diverging arm doesn't
decide the result type) and reports `.noreturn` only when EVERY arm diverges —
so a mixed match (some arms yield values, some diverge) infers the value type.

This unblocks ERR E1.5's `catch` match-body form (`x catch e == { case .A:
return …; else: raise e; }`), which desugars to an all-diverging match.

Regression: examples/225-match-diverging-arms.sx (all-diverging + mixed,
exit 134). Gates: zig build, zig build test, 263/263 examples.

examples/225-match-diverging-arms.sx

@@ -0,0 +1,40 @@
+// Regression for issue 0057: a match (`if subject == { case ... }`) whose arms
+// ALL diverge (each `return`s) used to fail LLVM verification (a `void` phi +
+// "terminator in the middle of a basic block") — lowerMatch emitted a
+// value-merge phi and a fallback `const` into arm blocks that had already
+// terminated. Now a fully-diverging match produces no merge phi, and a mixed
+// match (some arms diverge, some yield values) materializes the merge only
+// from the value-producing arms.
+
+#import "modules/std.sx";
+
+// All arms diverge — the match is `noreturn`, no merge phi.
+classify :: (n: s32) -> s32 {
+    if n == {
+        case 0: return 10;
+        case 1: return 20;
+        else:   return 90;
+    }
+    return 0;                       // unreachable
+}
+
+// Mixed: value arms + a diverging arm.
+pick :: (n: s32) -> s32 {
+    v := if n == {
+        case 0: 1;
+        case 1: return 100;         // diverging arm — no fallback const after its `ret`
+        else:   3;
+    };
+    return v + 5;
+}
+
+main :: () -> s32 {
+    r : s32 = 0;
+    r = r + classify(0);            // 10
+    r = r + classify(1);            // 20
+    r = r + classify(7);            // 90
+    r = r + pick(0);                // 1 + 5 = 6
+    r = r + pick(9);                // 3 + 5 = 8  (else arm)
+    print("match result: {}\n", r); // 10+20+90+6+8 = 134
+    return r;
+}