fix(0098): enum literal resolves against the unwrapped optional child; non-enum targets are diagnosed

lowerEnumLiteral resolved the variant against the raw destination type,
so any non-enum destination fell into resolveVariantValue's silent
return-0 tail with the enum_init stamped as the wrong type:

  - ?E destinations produced variant 0 mis-typed as the optional
    (observed as variant 0 OR null, layout-dependent);
  - builtin destinations (i64) silently became 0;
  - unknown variants of real enums silently became variant 0;
  - a destination-less literal panicked LLVM emission (unresolved
    type reached codegen).

Now: optional destinations unwrap to the child enum (the coercion
layer's .optional_wrap handles E -> ?E), and the remaining shapes are
diagnosed — unknown variant (with the variant list, via the new
emitBadEnumVariant twin of emitBadVariant), non-enum destination, and
no destination (cascade-guarded: silent when the destination's type
already failed to resolve and was reported).

Regression tests: examples/0183 (return/assign/reassign into ?Enum,
non-zero variants, null path) + examples/1169/1170 (each diagnostic);
all three FAIL on pre-fix master. zig build test 426/426;
tests/run_examples.sh 598/598.

examples/expected/0183-types-enum-literal-optional-target.exit

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

examples/expected/0183-types-enum-literal-optional-target.stderr

@@ -0,0 +1 @@
+

examples/expected/0183-types-enum-literal-optional-target.stdout

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

examples/expected/1169-diagnostics-enum-literal-bad-target.exit

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

examples/expected/1169-diagnostics-enum-literal-bad-target.stderr

@@ -0,0 +1,11 @@
+error: 'nonexistent' is not a variant of 'Platform' (variants are: ios, android_apk)
+  --> examples/1169-diagnostics-enum-literal-bad-target.sx:10:20
+   |
+10 |     a : Platform = .nonexistent;   // unknown variant: lists the real ones
+   |                    ^^^^^^^^^^^^
+
+error: enum literal '.foo' cannot type itself from non-enum destination 'i64'
+  --> examples/1169-diagnostics-enum-literal-bad-target.sx:11:15
+   |
+11 |     b : i64 = .foo;                // non-enum destination
+   |               ^^^^

examples/expected/1169-diagnostics-enum-literal-bad-target.stdout

@@ -0,0 +1 @@
+

examples/expected/1170-diagnostics-enum-literal-no-target.exit

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

examples/expected/1170-diagnostics-enum-literal-no-target.stderr

@@ -0,0 +1,5 @@
+error: enum literal '.ios' has no destination type to resolve against
+  --> examples/1170-diagnostics-enum-literal-no-target.sx:8:10
+   |
+ 8 |     c := .ios;
+   |          ^^^^

examples/expected/1170-diagnostics-enum-literal-no-target.stdout

@@ -0,0 +1 @@
+