refactor(ffi-linkage): Phase 8 pre-cutover — migrate keyword-neutral diagnostic tests

Migrate the two #foreign-bearing diagnostic tests whose assertions survive the
cutover, with INTENTIONAL snapshot regens (reviewed):
- 1172 (foreign-symbol-conflict): decl '#foreign libc "getenv"' → 'extern libc
  "getenv"'. Still tests the dup-C-symbol conflict; the 'foreign symbol already
  bound' message is the keyword-neutral INTERNAL wording (renamed to 'extern symbol'
  in Phase 9.1), so it persists — only the echoed source line + caret moved.
- 1228 (non-transitive C-import visibility): its identity was the #foreign≡extern
  equivalence lock, now historical (structural via the A→B gate + unified AST). The
  identifier 'c_foreign_abs' itself contained 'foreign' (would fail the Phase 9.4
  gate), so converted c.sx/b.sx/main to two foreign-free extern symbols
  (c_abs_one/c_abs_two); still pins per-symbol non-transitive visibility.

Reverted the orthogonal 0→1-byte empty-stdout normalization on 1228/1231 (known
writeGolden idempotency quirk, not a behavior change). Suite green (647/444).

examples/1172-diagnostics-foreign-symbol-conflict.sx

@@ -6,7 +6,7 @@
 
 libc :: #library "c";
 // std/process.sx already binds getenv as `-> *u8`; this view disagrees.
-getenv_opt :: (name: [:0]u8) -> ?[:0]u8 #foreign libc "getenv";
+getenv_opt :: (name: [:0]u8) -> ?[:0]u8 extern libc "getenv";
 
 main :: () -> i32 {
     p := getenv_opt("PATH");

examples/1228-ffi-extern-c-non-transitive.sx

@@ -1,19 +1,14 @@
 // `#import` is non-transitive for C-import functions: main imports b,
-// b imports c, so main must NOT see c's lib-less `#foreign` / `extern`
-// C functions directly. Referencing either is rejected by the
-// C-import visibility gate (lower/decl.zig `c_import_bare`).
-//
-// Equivalence lock (FFI-linkage Part B): the legacy `#foreign` form and
-// the new `extern` keyword are two spellings of the same lib-less
-// C-symbol import, so they must produce the SAME "C function not
-// visible" diagnostic — not the generic top-level-name wording. This
-// pins that equivalence as a prerequisite for migrating the fn-decl
-// `#foreign` path onto `extern`.
+// b imports c, so main must NOT see c's lib-less `extern` C functions
+// directly. Referencing either is rejected by the C-import visibility
+// gate (lower/decl.zig `c_import_bare`) with a C-specific "not visible"
+// diagnostic — not the generic top-level-name wording. Two distinct
+// extern symbols pin that the gate fires per-symbol.
 #import "modules/std.sx";
 #import "1228-ffi-extern-c-non-transitive/b.sx";
 
 main :: () -> i32 {
-    print("{}\n", c_foreign_abs(-3));
-    print("{}\n", c_extern_abs(-4));
+    print("{}\n", c_abs_one(-3));
+    print("{}\n", c_abs_two(-4));
     return 0;
 }

examples/1228-ffi-extern-c-non-transitive/b.sx

@@ -3,5 +3,5 @@
 #import "c.sx";
 
 b_use :: () -> i32 {
-    return c_foreign_abs(-1) + c_extern_abs(-2);
+    return c_abs_one(-1) + c_abs_two(-2);
 }

examples/1228-ffi-extern-c-non-transitive/c.sx

@@ -1,7 +1,5 @@
-// Two spellings of the same lib-less C-symbol import: the legacy
-// `#foreign` form and the new `extern` keyword. Both declare a C
-// function resolved at link time with no library reference. The
-// FFI-linkage migration (Part B) requires the two to be policed
-// IDENTICALLY by the non-transitive C-import visibility gate.
-c_foreign_abs :: (x: i32) -> i32 #foreign;
-c_extern_abs  :: (x: i32) -> i32 extern;
+// Two lib-less `extern` C-symbol imports: each declares a C function
+// resolved at link time with no library reference. Both are policed by
+// the non-transitive C-import visibility gate, per-symbol.
+c_abs_one :: (x: i32) -> i32 extern;
+c_abs_two :: (x: i32) -> i32 extern;

examples/expected/1172-diagnostics-foreign-symbol-conflict.stderr

@@ -1,5 +1,5 @@
 error: foreign symbol 'getenv' is already bound with a different signature; two views of one C symbol must declare identical types
-  --> examples/1172-diagnostics-foreign-symbol-conflict.sx:9:41
+  --> examples/1172-diagnostics-foreign-symbol-conflict.sx:9:61
    |
- 9 | getenv_opt :: (name: [:0]u8) -> ?[:0]u8 #foreign libc "getenv";
-   |                                         ^^^^^^^^^^^^^^^^^^^^^^^
+ 9 | getenv_opt :: (name: [:0]u8) -> ?[:0]u8 extern libc "getenv";
+   |                                                             ^

examples/expected/1228-ffi-extern-c-non-transitive.stderr

@@ -1,11 +1,11 @@
-error: C function 'c_foreign_abs' not visible; add #import for the module that declares it
-  --> examples/1228-ffi-extern-c-non-transitive.sx:16:19
+error: C function 'c_abs_one' not visible; add #import for the module that declares it
+  --> examples/1228-ffi-extern-c-non-transitive.sx:11:19
    |
-16 |     print("{}\n", c_foreign_abs(-3));
-   |                   ^^^^^^^^^^^^^
+11 |     print("{}\n", c_abs_one(-3));
+   |                   ^^^^^^^^^
 
-error: C function 'c_extern_abs' not visible; add #import for the module that declares it
-  --> examples/1228-ffi-extern-c-non-transitive.sx:17:19
+error: C function 'c_abs_two' not visible; add #import for the module that declares it
+  --> examples/1228-ffi-extern-c-non-transitive.sx:12:19
    |
-17 |     print("{}\n", c_extern_abs(-4));
-   |                   ^^^^^^^^^^^^
+12 |     print("{}\n", c_abs_two(-4));
+   |                   ^^^^^^^^^