Implement reader macros (#;, #|...|, #', #name) and #z3 demo

Reader macros in parser.sx spec, Python parser.py, and hand-written sx.js:
- #; datum comment: read and discard next expression
- #|...|  raw string: no escape processing
- #' quote shorthand: (quote expr)
- #name extensible dispatch: registered handler transforms next expression

#z3 reader macro demo (reader_z3.py): translates define-primitive
declarations from primitives.sx into SMT-LIB verification conditions.
Same source, two interpretations — bootstrappers compile to executable
code, #z3 extracts proof obligations.

48 parser tests (SX spec + Python), all passing. Rebootstrapped JS+Python.
Demo page at /plans/reader-macro-demo with side-by-side examples.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/tests/test_parser.py

@@ -235,3 +235,57 @@ class TestSerialize:
     def test_roundtrip(self):
         original = '(div :class "main" (p "hello") (span 42))'
         assert serialize(parse(original)) == original
+
+
+# ---------------------------------------------------------------------------
+# Reader macros
+# ---------------------------------------------------------------------------
+
+class TestReaderMacros:
+    """Test #; datum comment, #|...| raw string, and #' quote shorthand."""
+
+    def test_datum_comment_discards(self):
+        assert parse_all("#;(ignored) 42") == [42]
+
+    def test_datum_comment_in_list(self):
+        assert parse("(1 #;2 3)") == [1, 3]
+
+    def test_datum_comment_nested(self):
+        assert parse_all("#;(a (b c) d) 99") == [99]
+
+    def test_raw_string_basic(self):
+        assert parse('#|hello|') == "hello"
+
+    def test_raw_string_with_quotes(self):
+        assert parse('#|say "hi"|') == 'say "hi"'
+
+    def test_raw_string_with_backslashes(self):
+        assert parse('#|a\\nb|') == 'a\\nb'
+
+    def test_raw_string_empty(self):
+        assert parse('#||') == ""
+
+    def test_quote_shorthand_symbol(self):
+        assert parse("#'foo") == [Symbol("quote"), Symbol("foo")]
+
+    def test_quote_shorthand_list(self):
+        assert parse("#'(1 2 3)") == [Symbol("quote"), [1, 2, 3]]
+
+    def test_hash_at_eof_errors(self):
+        with pytest.raises(ParseError):
+            parse("#")
+
+    def test_unknown_reader_macro_errors(self):
+        with pytest.raises(ParseError, match="Unknown reader macro"):
+            parse("#x foo")
+
+    def test_extensible_reader_macro(self):
+        """Registered reader macros transform the next expression."""
+        from shared.sx.parser import register_reader_macro
+        register_reader_macro("upper", lambda expr: str(expr).upper())
+        try:
+            result = parse('#upper "hello"')
+            assert result == "HELLO"
+        finally:
+            from shared.sx.parser import _READER_MACROS
+            del _READER_MACROS["upper"]