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

A qualified generic type head `ns.Box(args)` was stripped to its bare name and
read from the last-wins `struct_template_map`, so the namespace qualifier never
selected the template author: `a.Box(s64)` and `b.Box(s64)` (two namespaces each
authoring a same-name `Box($T)` with different layouts) both instantiated the
global same-name template. The documented ambiguity escape hatch ("qualify it as
ns.Box") silently produced the wrong layout.

Select the template via the namespace edge (importer -> alias -> NamespaceTarget)
instead of the bare map, at both the .call and parameterized-type-expr head
sites. Two same-name templates instantiated with the same args would also collide
on the mangled name `Box__s64`, so tag the non-canonical author's mangled name
with its source (the canonical bare-map author keeps the untagged name -> no
churn for single-author generics).

Extract `buildGenericStructTemplate` so the bare registration and the new
namespace-qualified selection share one template builder.

Regression: examples/0772 — two namespaces each authoring Box($T) with different
layouts; ns_a.Box(s64) and ns_b.Box(s64) resolve to their own module's template
(sizes 8 and 16). Fail-before on 566de96 (a=16 b=16), pass-after (a=8 b=16).

examples/expected/0772-modules-qualified-generic-head-author.exit

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

examples/expected/0772-modules-qualified-generic-head-author.stderr

@@ -0,0 +1 @@
+

examples/expected/0772-modules-qualified-generic-head-author.stdout

@@ -0,0 +1,2 @@
+a=8 b=16
+pa.x=1 pb.x=10 pb.y=20