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/0770-modules-type-fn-head-non-transitive.sx

@@ -0,0 +1,23 @@
+// A type-returning FUNCTION head (`Make(s64)` where `Make :: ($T) -> Type`) is
+// NON-transitive even when a DIRECT flat import authors the same name as a
+// NON-function. `main` flat-imports `b.sx`; `b.sx` declares `Make :: 123` (a
+// value const, not a type-fn) AND flat-imports `c.sx`, whose `Make` IS the
+// type-returning function. The only TYPE-FN author of `Make` is two flat hops
+// away (main → b → c), so the bare `Make(s64)` head must emit the
+// "type 'Make' is not visible" diagnostic and poison — the visible 1-hop
+// `Make :: 123` const must NOT vouch for it.
+//
+// Regression (Phase E4 attempt-7, finding E4-type-fn-head-hidden-by-visible-
+// nonfn): before `headFnLeak` decided visibility from the ELIGIBLE FUNCTION
+// authors it used the module-scope NAME predicate (`isNameVisible`), which the
+// visible non-fn `Make :: 123` satisfied — so the global `fn_ast_map` type-fn
+// silently instantiated and `size_of(Make(s64))` printed 8 at exit 0 instead of
+// the visibility diagnostic.
+
+#import "modules/std.sx";
+#import "0770-modules-type-fn-head-non-transitive/b.sx";
+
+main :: () -> s32 {
+    print("size={}\n", size_of(Make(s64)));
+    0
+}

examples/0770-modules-type-fn-head-non-transitive/b.sx

@@ -0,0 +1,6 @@
+// The directly-imported (1-hop) author of the NAME `Make` — but as a value
+// const, NOT a type-returning function. It flat-imports c.sx (where the real
+// `Make` type-fn lives, two hops from a file that imports b.sx). A same-name
+// non-function must not vouch for the 2-hop type-fn head.
+#import "c.sx";
+Make :: 123;

examples/0770-modules-type-fn-head-non-transitive/c.sx

@@ -0,0 +1,5 @@
+// The real type-returning function `Make`, two flat hops from a file that
+// imports b.sx. A file importing only b.sx must NOT see this head.
+Make :: ($T: Type) -> Type {
+    return struct { x: T; };
+}

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 @@
+