fix(stdlib/E4): qualified generic alias head a.Box(..) selects the namespace author

The const-decl alias-registration path treated a qualified generic head
(`ABox :: a.Box(s64)`) only as a gate exemption, then read the bare last-wins
`struct_template_map` — so `ABox` and `BBox` both instantiated whichever
same-name template won globally (both size 16). attempt-9 routed the annotation
head sites through `qualifiedStructTemplate`; this applies the same selection to
the two alias-registration branches (.call and .parameterized_type_expr) before
the bare fallback, and extracts the shared instantiate-and-register logic into
`registerGenericStructAlias`.

ABox :: a.Box(s64) now resolves to a's template (size 8); BBox :: b.Box(s64) to
b's (size 16). Regression 0773 pins it (fail-before alias a=16 b=16, after a=8
b=16).

examples/0773-modules-qualified-generic-alias-author/a.sx

@@ -0,0 +1,2 @@
+// Author A's generic `Box` — one s64 field (size 8).
+Box :: struct($T: Type) { x: T; }

examples/0773-modules-qualified-generic-alias-author/b.sx

@@ -0,0 +1,3 @@
+// Author B's generic `Box` — two s64 fields (size 16). Same template NAME as
+// A's, different layout: the qualified alias head must select by namespace author.
+Box :: struct($T: Type) { x: T; y: T; }