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.

src/ir/conversions.zig

@@ -36,6 +36,7 @@ pub const CoercionResolver = struct {
         optional_unwrap, // ?T → concrete (narrowing)
         optional_to_bool_reject, // ?T → bool (no presence-test coercion; diagnostic)
         void_to_optional, // void (null literal) → ?T
+        optional_to_optional, // ?A → ?B (presence-preserving payload coercion)
         optional_wrap, // concrete → ?T
         erase_protocol, // concrete → protocol value
         int_to_float,
@@ -114,6 +115,16 @@ pub const CoercionResolver = struct {
                 if (child_ty == dst_ty or (dst_ty.isBuiltin() and child_ty.isBuiltin())) {
                     return .optional_unwrap;
                 }
+                // ?A → ?B: a presence-preserving payload coercion. Without a
+                // dedicated arm this fell to `.optional_wrap` (dst is optional),
+                // which unwrapped the SOURCE optional unconditionally and re-
+                // wrapped it as always-present — turning a null `?i32` into a
+                // present `?i64` carrying the zero payload (issue 0180: generic
+                // `??` returning the wrong fallback). Only meaningful when the
+                // children differ (same-type optionals are `.no_op` already).
+                if (!dst_ty.isBuiltin() and self.l.module.types.get(dst_ty) == .optional) {
+                    return .optional_to_optional;
+                }
             }
         }