fix: thread optional child type into ?? struct-literal default (issue 0166)

The RHS of a null-coalesce was lowered with no target type, so a bare
struct literal default (x ?? .{ ... }) produced a struct_init with
.ty == .unresolved that panicked in emitStructInit. lowerNullCoalesce
now saves self.target_type, sets it to the optional's resolved child
before lowering nc.rhs, and restores it (leak-free). Verified across
struct/slice/enum/tuple/protocol/nested-optional/generic child types by
3 adversarial reviews.

Regression: examples/optionals/0912-null-coalesce-struct-literal.sx.
Filed adjacent pre-existing bug 0172 (?? on a non-optional lhs panics).

examples/optionals/0912-null-coalesce-struct-literal.sx

@@ -0,0 +1,30 @@
+// Struct literal as the default of `??` (null-coalesce).
+// Regression (issue 0166): a bare `.{ ... }` default was lowered with no
+// target type, so its `struct_init.ty` stayed `.unresolved` and panicked at
+// LLVM emission ("unresolved type reached LLVM emission"). The fix threads the
+// optional's child type `T` into the RHS lowering, so the literal resolves to
+// `T`. Covers both runtime paths: default TAKEN (lhs null) and NOT taken (lhs
+// present), plus a nested struct-literal default and an all-defaulted `.{}`.
+#import "modules/std.sx";
+
+Inner :: struct { x: i64 = 0; }
+T :: struct { a: i64 = 7; b: i64 = 3; inner: Inner; }
+
+mk :: (give: bool) -> ?T {
+    if give { return .{ a = 1, b = 2, inner = .{ x = 4 } }; }
+    return null;
+}
+
+main :: () {
+    // Default NOT taken — lhs has a value.
+    present := mk(true) ?? .{ a = 9, inner = .{ x = 99 } };
+    print("{} {} {}\n", present.a, present.b, present.inner.x);
+
+    // Default TAKEN — lhs is null, struct-literal default selected.
+    absent := mk(false) ?? .{ a = 9, inner = .{ x = 99 } };
+    print("{} {} {}\n", absent.a, absent.b, absent.inner.x);
+
+    // All-defaulted `.{}` default.
+    empty := mk(false) ?? .{};
+    print("{} {} {}\n", empty.a, empty.b, empty.inner.x);
+}

examples/optionals/expected/0912-null-coalesce-struct-literal.exit

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

examples/optionals/expected/0912-null-coalesce-struct-literal.stderr

@@ -0,0 +1 @@
+

examples/optionals/expected/0912-null-coalesce-struct-literal.stdout

@@ -0,0 +1,3 @@
+1 2 4
+9 3 99
+7 3 0