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/expected/0916-optionals-generic-null-coalesce-fallback.exit

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

examples/optionals/expected/0916-optionals-generic-null-coalesce-fallback.stderr

@@ -0,0 +1 @@
+

examples/optionals/expected/0916-optionals-generic-null-coalesce-fallback.stdout

@@ -0,0 +1,7 @@
+a=99
+b=5
+c=7
+d=42
+e=123
+f=1 2
+g=8 9

examples/optionals/expected/0917-optionals-alias-null-coalesce-struct-default.exit

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

examples/optionals/expected/0917-optionals-alias-null-coalesce-struct-default.stderr

@@ -0,0 +1 @@
+

examples/optionals/expected/0917-optionals-alias-null-coalesce-struct-default.stdout

@@ -0,0 +1,3 @@
+a=7 8
+b=1 2
+c=0 0

examples/optionals/expected/0918-optionals-vardecl-optional-to-optional.exit

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

examples/optionals/expected/0918-optionals-vardecl-optional-to-optional.stderr

@@ -0,0 +1 @@
+

examples/optionals/expected/0918-optionals-vardecl-optional-to-optional.stdout

@@ -0,0 +1,10 @@
+widen-null: absent
+widen-pres: 7
+narrow-null: absent
+narrow-pres: 9
+i2f-null: absent
+wrap: 5
+arr2slice-pres: len=3 first=10
+arr2slice-null: absent
+erase-pres: area=16
+erase-null: absent