refactor(ffi-linkage): Phase 9.3-src — purge 'foreign' from src/ comments + a user-facing diagnostic

Reword every 'foreign' comment to the extern/runtime-class vocabulary matching the
renamed identifiers (foreign call→extern call, foreign class→runtime class, foreign
path→runtime path, the #foreign-literal comment mentions → extern, etc.). Also fixes
two USER-FACING issues: the 'expected … #foreign … after type annotation' parse error
no longer advertises the removed keyword, and the Android 'no #jni_main' help
diagnostic now shows '#jni_class(…) extern' instead of the rejected '#foreign
#jni_class'. Removed the now-dead prefix-#foreign-vs-postfix conflict branch in
parseRuntimeClassDecl (the caller rejects #foreign before it runs).

src/ now contains 'foreign' ONLY in the hash_foreign token machinery + its 4
rejection messages — the deprecation mechanism (kept per the 9.0 recommendation; the
message MUST name #foreign to guide migration). Snapshot-neutral; suite green
(646 corpus / 444 unit, 0 failed).

src/main.zig

@@ -270,7 +270,7 @@ pub fn main(init: std.process.Init) !void {
         defer c_handle.unload(io);
         timer.record("c-import");
 
-        // dlopen #library dependencies so JIT can resolve foreign symbols.
+        // dlopen #library dependencies so JIT can resolve extern symbols.
         // Program-owned dylibs (the #import c unit first, then #library
         // deps in declaration order) also become PRIORITY search targets
         // for the JIT, consulted before the process-wide fallback.