issues/0066: RESOLVED — match-value arms lowered against result type

A value-position match's arms are now lowered with `target_type` set to
the merge's `result_type`, so positive and negated integer literals pick
the same width. Fixes the `PHI node operands are not the same type as the
result` failure for `if n == { case 0: 100; else: -1; }`-style returns.

Regression: examples/0043-basic-match-value-mixed-width.sx.
Gates: zig build, zig build test, run_examples.sh -> 345 passed.

src/ir/lower.zig

@@ -4497,7 +4497,13 @@ pub const Lowering = struct {
             }
 
             if (has_value_merge) {
+                // Lower the arm body against the merge's result type so literals
+                // (and negated literals) in the arm pick the right width — the
+                // phi operands must all match `result_type` (issue 0066).
+                const saved_arm_target = self.target_type;
+                self.target_type = result_type;
                 const maybe_v = self.lowerBlockValue(arm.body);
+                self.target_type = saved_arm_target;
                 self.current_match_tags = saved_match_tags;
                 self.scope = old_scope;
                 arm_scope.deinit();