fix(stdlib/E4): bare generic head selects visible author; qualified missing-member diagnoses

E4 non-transitive type rule had two generic-head author-selection holes:

#1 A BARE generic struct head / alias with a single bare-VISIBLE author still
   instantiated a NON-visible 2-flat-hop same-name template, because the
   `.unregistered` gate arm fell through to the global last-wins
   `struct_template_map` winner. Add `bareVisibleStructTemplate`: after the
   visibility gate passes, select the source-keyed template authored by the
   single bare-visible author (own-wins, else the one 1-hop flat author) and
   instantiate THAT instead of the global map's last-wins entry. Null (→ the
   global map, byte-identical) when the visible author IS the canonical one
   (the common single-author case) or the picture isn't a clean single author.
   Applied at every bare generic-struct head/alias site (annotation `.call` /
   `.parameterized_type_expr`, alias-registration `.call` /
   `.parameterized_type_expr`, array-literal head).

#2 A QUALIFIED head `a.Box(..)` whose namespace `a` authors no member `Box`
   silently fell back to the bare global template, instantiating an unrelated
   module's `Box`. Add `qualifiedMemberMissing`: a qualified head whose known
   namespace lacks the member now emits "namespace 'a' has no member 'Box'" and
   poisons with `.unresolved`; a qualified head NEVER reaches the bare global map.

Regressions: 0774 (bare head + bare alias, 2-hop same-name → size=8 alias=8,
fail-before 16 16); 0775 (qualified missing member → diagnostic + exit 1,
fail-before size=16 exit 0).

examples/0774-modules-bare-generic-head-visible-author.sx

@@ -0,0 +1,28 @@
+// A BARE generic struct head (`Box(s64)`) and a BARE generic alias
+// (`ABox :: Box(s64)`) must instantiate the template authored by the single
+// bare-VISIBLE author — this file's own author or a DIRECT (1-hop) flat import —
+// NOT the global last-wins `struct_template_map`, which a NON-visible 2-flat-hop
+// same-name template can win.
+//
+// `b.sx` declares a one-field `Box($T)` (size 8) and itself flat-imports `c.sx`,
+// which declares a two-field `Box($T)` (size 16). This file flat-imports ONLY
+// `b.sx`, so `b.Box` is one flat hop away (visible) and `c.Box` is two hops away
+// (NOT bare-visible, mirrors 0764/0706). The bare head `Box(s64)` and the bare
+// alias `ABox :: Box(s64)` must both select `b.Box` (size 8).
+//
+// Regression (Phase E4 finding #1): before the bare head/alias consulted the
+// source-keyed visible author, both fell through the `.unregistered` gate arm to
+// the global last-wins template and instantiated the non-visible `c.Box`
+// (size 16). Fail-before printed `size=16 alias=16`.
+
+#import "modules/std.sx";
+#import "0774-modules-bare-generic-head-visible-author/b.sx";
+
+ABox :: Box(s64);
+
+main :: () -> s32 {
+    x : Box(s64) = .{ x = 1 };
+    a : ABox = .{ x = 2 };
+    print("size={} alias={} x={} a={}\n", size_of(Box(s64)), size_of(ABox), x.x, a.x);
+    0
+}

examples/0774-modules-bare-generic-head-visible-author/b.sx

@@ -0,0 +1,11 @@
+// The bare-VISIBLE author: a one-field generic `Box` (size 8). `b.sx` itself
+// flat-imports `c.sx`, so `b_make`'s `Box(s64)` resolves here (the head is one
+// flat hop away in this module) — but a file that imports b.sx reaches `c.Box`
+// only at two hops, so it must NOT win the bare head in the importer.
+Box :: struct($T: Type) { x: T; }
+
+#import "c.sx";
+
+b_make :: () -> Box(s64) {
+    .{ x = 99 }
+}

examples/0774-modules-bare-generic-head-visible-author/c.sx

@@ -0,0 +1,5 @@
+// The NON-visible 2-flat-hop author: a two-field generic `Box` (size 16). Same
+// template NAME as b's, different layout. It wins the global last-wins
+// `struct_template_map`, so the bare head in a file importing only b.sx must NOT
+// pick it.
+Box :: struct($T: Type) { x: T; y: T; }

examples/0775-modules-qualified-generic-missing-member.sx

@@ -0,0 +1,23 @@
+// A QUALIFIED generic head `a.Box(s64)` where namespace `a` exists but authors
+// NO member named `Box` must DIAGNOSE the missing member — never silently fall
+// back to the bare last-wins `struct_template_map` and instantiate an unrelated
+// module's same-name `Box`.
+//
+// `a.sx` authors only `Other` (no `Box`); `b.sx` authors a generic `Box($T)`.
+// The qualified reference `a.Box(s64)` must report that `a` has no member `Box`,
+// NOT resolve to `b.Box`.
+//
+// Regression (Phase E4 finding #2): before the qualified head path diagnosed the
+// missing member, `qualifiedStructTemplate` returned null and the code fell
+// through to the bare global template, silently instantiating `b.Box`
+// (`size=16 x=1 y=2`, exit 0).
+
+#import "modules/std.sx";
+a :: #import "0775-modules-qualified-generic-missing-member/a.sx";
+b :: #import "0775-modules-qualified-generic-missing-member/b.sx";
+
+main :: () -> s32 {
+    x : a.Box(s64) = .{ x = 1, y = 2 };
+    print("{}\n", x.x);
+    0
+}

examples/0775-modules-qualified-generic-missing-member/a.sx

@@ -0,0 +1,3 @@
+// Namespace `a` authors ONLY `Other` — no `Box`. A qualified `a.Box(..)` head
+// must diagnose the missing member, not resolve to another module's `Box`.
+Other :: struct { z: s64; }

examples/0775-modules-qualified-generic-missing-member/b.sx

@@ -0,0 +1,4 @@
+// An unrelated module's generic `Box($T)` (two fields, size 16). The qualified
+// head `a.Box(..)` must NOT silently resolve to this template via the bare
+// global last-wins map.
+Box :: struct($T: Type) { x: T; y: T; }

examples/expected/0774-modules-bare-generic-head-visible-author.exit

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

examples/expected/0774-modules-bare-generic-head-visible-author.stdout

@@ -0,0 +1 @@
+size=8 alias=8 x=1 a=2

examples/expected/0775-modules-qualified-generic-missing-member.exit

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

examples/expected/0775-modules-qualified-generic-missing-member.stderr

@@ -0,0 +1,5 @@
+error: namespace 'a' has no member 'Box'
+  --> examples/0775-modules-qualified-generic-missing-member.sx:20:9
+   |
+20 |     x : a.Box(s64) = .{ x = 1, y = 2 };
+   |         ^^^^^^^^^^