fix(stdlib/E4): kind-aware type-fn head gate (non-fn must not vouch)

The type-fn head visibility check (`headFnLeak`) used the module-scope
NAME predicate `isNameVisible`, so a same-name 1-hop NON-function (a value
const `Make :: 123`) reported the name "visible" and let the global
`fn_ast_map` type-fn — whose real author is 2 flat hops away — silently
instantiate. `size_of(Make(s64))` printed 8 at exit 0 instead of a
visibility diagnostic.

Decide visibility from the ELIGIBLE FUNCTION authors directly reachable
from the use site (`flatFnAuthorVisible`, mirroring `flatFnAuthorAmbiguous`'s
fn-only author view): visible iff the own author or a 1-hop flat-import
author is a `fn_decl`. A non-function does not vouch. Guarded to fall open
when the import facts aren't wired (comptime / directory imports), mirroring
`headTypeGate`. Own / scope-local / 1-hop / directly-imported type-fn heads
still resolve; 0769 ambiguity unchanged.

Regression: examples/0770-modules-type-fn-head-non-transitive (main → b
[`Make :: 123` + flat-imports c] → c [`Make :: ($T) -> Type`]); the bare
`Make(s64)` head emits "type 'Make' is not visible", exit 1.

examples/expected/0770-modules-type-fn-head-non-transitive.exit

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

examples/expected/0770-modules-type-fn-head-non-transitive.stderr

@@ -0,0 +1,5 @@
+error: type 'Make' is not visible; #import the module that declares it
+  --> examples/0770-modules-type-fn-head-non-transitive.sx:21:32
+   |
+21 |     print("size={}\n", size_of(Make(s64)));
+   |                                ^^^^

examples/expected/0770-modules-type-fn-head-non-transitive.stdout

@@ -0,0 +1 @@
+