lang: array-typed '::' consts as immutable globals (PLAN-CONST-AGG step 1)

K : [4]s64 : .[...] and the untyped A :: .[1, 2, 3] register as
is_const globals: one storage, reads GEP it, dead-global elimination
drops unused ones, source-aware reads come free via selectGlobalAuthor.

- registerConstArrayGlobal (scanDecls pass 2): typed via the annotation
  (array-ness + dimension/count checked), untyped via element-type
  unification — all ints s64; ANY float promotes the element type to
  f64 with ints converting exactly; bool/string homogeneous; a
  non-numeric mix or non-inferable element asks for an annotation.
- constExprValue converts int elements into float destinations exactly
  (the int+float promotion rule, element-wise).
- emitGlobals marks is_const globals LLVMSetGlobalConstant — also flips
  the comptime-backed #run globals and __sx_default_context to
  'constant' (37 pinned IR snapshots regenerated; runtime unchanged).
- Element shapes: nested arrays, struct elements, strings, bools.
  Non-constant elements / dim mismatch / mixed types diagnose loudly.

Examples: 0177 (feature matrix incl. @K reads through *[4]s64 — needs
the 0117 fix), 1159/1160/1161 (diagnostics), 0837 repointed to values.

examples/expected/1009-errors-catch.ir

@@ -1,5 +1,5 @@
 
-@__sx_default_context = internal global { { ptr, ptr, ptr }, ptr } { { ptr, ptr, ptr } { ptr null, ptr @__thunk_CAllocator_Allocator_alloc, ptr @__thunk_CAllocator_Allocator_dealloc }, ptr null }
+@__sx_default_context = internal constant { { ptr, ptr, ptr }, ptr } { { ptr, ptr, ptr } { ptr null, ptr @__thunk_CAllocator_Allocator_alloc, ptr @__thunk_CAllocator_Allocator_dealloc }, ptr null }
 @str = private unnamed_addr constant [2 x i8] c"0\00", align 1
 @str.125 = private unnamed_addr constant [2 x i8] c"0\00", align 1
 @str.126 = private unnamed_addr constant [5 x i8] c"true\00", align 1