feat(ffi-linkage): lower extern data globals (Phase 1.2d) — Phase 1 complete

Parser: a 'kw_extern' branch in the var-decl-with-type-annotation path
(beside #foreign) parses 'name : type extern [LIB] ["csym"];' into
VarDecl.is_extern/extern_lib/extern_name; the trailing diagnostic now
lists 'extern'. Lowering: registerTopLevelGlobal uses
extern_name orelse foreign_name orelse name for the C symbol and sets
is_extern = is_foreign or is_extern; globalInitValue returns null (no
initializer) for extern globals too.

examples/1225 green: '__stdinp : *void extern;' lowers to
'@__stdinp = external global ptr'; @__stdinp reads non-null. Suite
green (636 corpus / 443 unit).

Phase 1 done: extern functions (bare + rename) and data globals (bare +
rename) all work, behavior-equivalent to the matching #foreign form.
export (Phase 2), aggregates (Phase 3), docs + A->B gate (Phase 4)
remain. green commit.

src/parser.zig

@@ -449,7 +449,35 @@ pub const Parser = struct {
             } });
         }
 
-        return self.fail("expected ':', '=', ';' or '#foreign' after type annotation");
+        if (self.current.tag == .kw_extern) {
+            // name : type extern [LIB] ["csym"];   (extern data global — the
+            // extern-named counterpart of `#foreign`; resolved at link time)
+            self.advance();
+            var ext_lib: ?[]const u8 = null;
+            if (self.current.tag == .identifier) {
+                ext_lib = self.tokenSlice(self.current);
+                self.advance();
+            }
+            var ext_name: ?[]const u8 = null;
+            if (self.current.tag == .string_literal) {
+                const raw = self.tokenSlice(self.current);
+                ext_name = raw[1 .. raw.len - 1];
+                self.advance();
+            }
+            try self.expect(.semicolon);
+            return try self.createNode(start_pos, .{ .var_decl = .{
+                .name = name,
+                .name_span = name_span,
+                .type_annotation = type_node,
+                .value = null,
+                .is_extern = true,
+                .extern_lib = ext_lib,
+                .extern_name = ext_name,
+                .is_raw = name_is_raw,
+            } });
+        }
+
+        return self.fail("expected ':', '=', ';', '#foreign', or 'extern' after type annotation");
     }
 
     fn parseTypeExpr(self: *Parser) anyerror!*Node {