feat(ffi-linkage): add kw_extern/kw_export tokens (Phase 0.0)

Lex 'extern' and 'export' as keywords beside 'callconv': new token.Tag
variants + keywords StaticStringMap entries + LSP semantic-token keyword
classification. Adds a 'lex linkage keywords' unit test.

Tokens only — parser/AST plumbing and lowering land in later phases.
Corpus sweep confirmed no .sx identifier collides with the new reserved
words. lock commit per the cadence rule.

src/lexer.zig

@@ -537,6 +537,15 @@ test "lex keywords" {
     }
 }
 
+test "lex linkage keywords" {
+    // extern / export are keywords (FFI-linkage stream), lexed beside callconv.
+    var lex = Lexer.init("callconv extern export");
+    const expected = [_]Tag{ .kw_callconv, .kw_extern, .kw_export };
+    for (expected) |exp| {
+        try std.testing.expectEqual(exp, lex.next().tag);
+    }
+}
+
 test "lex type-like identifiers" {
     // i32, u8, bool, string are identifiers, not keywords
     var lex = Lexer.init("i32 u8 bool string");