A module-global initialized with an enum literal silently zero-initialized
to the first tag (`chosen : Color = .green` read back as `.red`), and an
enum tag inside a global array/struct was rejected as non-constant. The
constant serializer had no enum-literal arm.
Add `Lowering.constEnumLiteral`: serialize an enum literal to a
`ConstantValue.int` holding the variant's tag value, resolved against the
destination enum type and respecting explicit variant values; the global's
type drives the backing width at emit time. Wired into `globalInitValue`
(scalar global) and `constExprValue` (array element / struct field / nested
aggregate). A non-enum destination or unknown variant is diagnosed loudly,
never silently zero-initialized. The compiler-injected OS/ARCH globals now
serialize to their real `.unknown` tag (6 / 4); runtime reads are unchanged
(they resolve through comptime_constants), so only the static initializer in
the pinned .ir snapshots changes.
Remove the silent `func_ref => orelse LLVMConstNull` fallbacks in the LLVM
constant emitters: aggregate func_ref leaves carry a `require_resolved` flag
(transient null in Pass 0, loud diagnostic if still unresolved in the
Pass-1.5 re-emit), a top-level func_ref global is resolved in
initVtableGlobals, and the comptime (#run) path bails loudly instead of
emitting a null function pointer.
Regression: examples/0139-types-global-enum-literal-init.sx (scalar, array,
struct field, explicit-value enum u16 stride, struct-array with enum field);
negative: examples/1127-diagnostics-global-enum-literal-bad-variant.sx.
Mark issue 0082 RESOLVED.
