WGSL: document a known limitation of the nested-constituent fold

A reviewer-identified edge case: an aggregate that is both a nested constituent
and used directly (a named `static const` array shared as an element of another
`static const` array and also independently runtime-indexed) is not folded
inline, so it stays a separate `var<private>` declaration that the enclosing
converted array's initializer references by name -- invalid WGSL. The shape is
not constructible from typical anonymous nested literals and is not a regression
(such a constant was rejected before this change too, as a runtime-indexed
`const`). Record it where the fold decision is made.

Co-authored-by: Harsh Aggarwal <haaggarwal@nvidia.com>

Author: nv-slang-bot[bot]

source/slang/slang-emit-wgsl.cpp

@@ -1426,6 +1426,14 @@ void WGSLSourceEmitter::emitCallArg(IRInst* inst)
 // of a larger aggregate, never used directly (e.g. it is not the base of a runtime GetElement).
 // Such a constituent must be inlined into its enclosing aggregate's initializer; an aggregate
 // used directly is the outermost one and must stay a declaration.
+//
+// Known limitation: an aggregate that is BOTH a nested constituent AND used directly (e.g. a
+// named `static const` array shared as an element of another `static const` array and also
+// independently runtime-indexed) returns false here, so it stays a separate `var<private>`
+// declaration that the enclosing converted array's initializer then references by name —
+// invalid WGSL. This shape is not constructible from typical anonymous nested literals (whose
+// inner Make* insts are used only as the parent's operands); fully handling it would require
+// inlining a duplicate copy of the constituent into the enclosing initializer.
 static bool isOnlyUsedAsAggregateConstituent(IRInst* inst)
 {
     bool hasUse = false;