fix: presence-preserving optional->optional coercion (issue 0180)

The generic-?? wrong-fallback was not in lowerNullCoalesce: coercing
?A -> ?B (differing payload, e.g. the ?i32->?i64 call-arg coercion when
instantiating unwrap_or(99, ?i32)) routed through .optional_wrap, which
unconditionally unwrapped the source and re-wrapped as ALWAYS-PRESENT, so
a null became present-zero everywhere (args, returns, field init,
var-decl, ??). Add a CoercionPlan.optional_to_optional (conversions.zig)
+ a presence-preserving arm in coerceMode (coerce.zig): has_value ->
present: unwrap+coerce-child+wrap-present; absent: constNull(dst); merge
via a dst_ty block param. lowerVarDecl gains a !src_is_optional guard so
an annotated x : ?B = <?A> routes through the same arm (also makes
aggregate-payload var-decl ?[3]i64->?[]i64 / ?Concrete->?Protocol work).

Alias-optional struct-literal default already works (grouping + 0166);
a 1-tuple default ?(i32,) ?? 5 now emits a clean diagnostic instead of an
LLVM PHI abort (no implicit scalar->1-tuple coercion per spec).

Regressions: optionals/0916 (generic ??), 0917 (alias struct default),
0918 (var-decl optional->optional), diagnostics/1202 (1-tuple default) +
a conversions.test.zig unit test. Verified by 3 adversarial reviews,
suite 798/0.

examples/optionals/0917-optionals-alias-null-coalesce-struct-default.sx

@@ -0,0 +1,28 @@
+// `??` on an alias-of-optional with a struct-literal default — present + null.
+//
+// Regression (issue 0180): `?Opt ?? S.{...}` where `Opt :: ?S` must resolve
+// the alias child to the same struct TypeId on both branches and lower the
+// struct-literal default against that resolved child (issue-0166 threading),
+// so the present-payload and the default share the merge TypeId. Both the
+// null path (default) and the present path (the payload) are exercised.
+#import "modules/std.sx";
+
+S :: struct { a: i64 = 0; b: i64 = 0; }
+Opt :: ?S;
+
+main :: () {
+    // null lhs -> the struct-literal default.
+    o : Opt = null;
+    x := o ?? S.{ a = 7, b = 8 };
+    print("a={} {}\n", x.a, x.b);
+
+    // present lhs -> the payload, default ignored.
+    o2 : Opt = S.{ a = 1, b = 2 };
+    y := o2 ?? S.{ a = 7, b = 8 };
+    print("b={} {}\n", y.a, y.b);
+
+    // untyped `.{}` default against the alias child (issue-0166 path).
+    o3 : Opt = null;
+    z := o3 ?? .{};
+    print("c={} {}\n", z.a, z.b);
+}