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/c_import.zig

@@ -309,7 +309,7 @@ pub fn processCImport(
 // Native C compilation (compile to .o, not LLVM module)
 // ---------------------------------------------------------------------------
 
-// ── #foreign ref validation ───────────────────────────────────────────
+// ── extern ref validation ───────────────────────────────────────────
 
 fn collectExternRefTargets(valid: *std.StringHashMap(void), decls: []const *Node) !void {
     for (decls) |d| {
@@ -349,7 +349,7 @@ fn checkExternRefs(valid: *const std.StringHashMap(void), decls: []const *Node,
     }
 }
 
-/// Validate every `#foreign <ref>` in the merged program: the ref must
+/// Validate every `extern <ref>` in the merged program: the ref must
 /// name a `#library` constant or a NAMED `#import c` unit. Refs are
 /// author-side module-local names, but modules merge flat or
 /// namespaced into one tree, so existence is checked program-wide.