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/core.zig

@@ -299,7 +299,7 @@ pub const Compilation = struct {
     pub fn lowerToIR(self: *Compilation) !ir.Module {
         const root = self.resolved_root orelse self.root orelse return ir.Module.init(self.allocator);
 
-        // Every `#foreign <ref>` must name a #library constant or a named
+        // Every `extern <ref>` must name a #library constant or a named
         // `#import c` unit — a typo'd ref otherwise resolves silently
         // through whatever image happens to carry the symbol.
         try c_import.validateExternRefs(self.allocator, root, &self.diagnostics);