fix(stdlib/E4): carry full author outcome through the bare-TYPE gate (ambiguity at every site)

attempt-4 gated every bare-type-reference site for VISIBILITY via a boolean
leak-check that only caught not-visible and DROPPED the ambiguous outcome, so two
DIRECT flat same-name type authors (the 0755/0105 ambiguity case) fell through to
a global findByName / struct_template_map pick at the non-leaf sites.

Unified author-outcome fix (one path, every site consumes it):

- flatTypeAuthorCount: ≥2 distinct flat authors that do NOT all collapse onto one
  shared TypeId are now `.ambiguous` even when none carries a concrete TypeId yet —
  two same-name GENERIC TEMPLATES (template name registered in no findByName slot)
  are a genuine collision, exactly like two registered structs. Identical-target
  authors (diamond import / two aliases onto the same target) still collapse to
  `.one`, so all valid cases stay byte-identical.

- headTypeGate: the complete source-aware author outcome (.proceed / .resolved /
  .ambiguous / .not_visible) for an unqualified bare TYPE head, emitting the loud
  ambiguity diagnostic (consistent with the leaf / 0755) or the not-visible
  diagnostic. headTypeLeak is now its poison-vs-proceed projection, so every head /
  instantiation / alias-decl / match site poisons on ambiguity with the right
  message. Reflection / type-arg and array/vector-literal identifier heads consume
  `.resolved` to use the source-keyed TypeId, never a global findByName pick.

Regression examples/0767: size_of(Thing) / Nums.[1,2] / Box(s64) / t:Type=Thing /
case Thing: with two direct flat same-name authors each emit the ambiguity
diagnostic, exit 1 (fail-before on bb8f7dc: exit 0 / cascade). 0763/0764/0765/0766
/0755/0706/0544/0105 + FFI byte-identical. README: bare-type ambiguity is enforced
at every reference site.

examples/expected/0767-modules-ambiguous-bare-type-forms.exit

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

examples/expected/0767-modules-ambiguous-bare-type-forms.stderr

@@ -0,0 +1,29 @@
+error: type 'Thing' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0767-modules-ambiguous-bare-type-forms.sx:33:19
+   |
+33 |     sz := size_of(Thing);
+   |                   ^^^^^
+
+error: type 'Nums' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0767-modules-ambiguous-bare-type-forms.sx:34:11
+   |
+34 |     xs := Nums.[1, 2];
+   |           ^^^^
+
+error: type 'Box' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0767-modules-ambiguous-bare-type-forms.sx:35:9
+   |
+35 |     x : Box(s64) = .{ v = 3 };
+   |         ^^^^^^^^
+
+error: type 'Thing' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0767-modules-ambiguous-bare-type-forms.sx:36:16
+   |
+36 |     t : Type = Thing;
+   |                ^^^^^
+
+error: type 'Thing' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0767-modules-ambiguous-bare-type-forms.sx:26:14
+   |
+26 |         case Thing: 1;
+   |              ^^^^^

examples/expected/0767-modules-ambiguous-bare-type-forms.stdout

@@ -0,0 +1 @@
+