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/ref/test-parser.sx

@@ -220,3 +220,40 @@
   (deftest "roundtrip nested"
     (assert-equal "(a (b c))"
       (sx-serialize (first (sx-parse "(a (b c))"))))))
+
+
+;; --------------------------------------------------------------------------
+;; Reader macros
+;; --------------------------------------------------------------------------
+
+(defsuite "reader-macros"
+  (deftest "datum comment discards expr"
+    (assert-equal (list 42) (sx-parse "#;(ignored) 42")))
+
+  (deftest "datum comment in list"
+    (assert-equal (list (list 1 3)) (sx-parse "(1 #;2 3)")))
+
+  (deftest "datum comment discards nested"
+    (assert-equal (list 99) (sx-parse "#;(a (b c) d) 99")))
+
+  (deftest "raw string basic"
+    (assert-equal (list "hello") (sx-parse "#|hello|")))
+
+  (deftest "raw string with quotes"
+    (assert-equal (list "say \"hi\"") (sx-parse "#|say \"hi\"|")))
+
+  (deftest "raw string with backslashes"
+    (assert-equal (list "a\\nb") (sx-parse "#|a\\nb|")))
+
+  (deftest "raw string empty"
+    (assert-equal (list "") (sx-parse "#||")))
+
+  (deftest "quote shorthand symbol"
+    (let ((result (first (sx-parse "#'foo"))))
+      (assert-equal "quote" (symbol-name (first result)))
+      (assert-equal "foo" (symbol-name (nth result 1)))))
+
+  (deftest "quote shorthand list"
+    (let ((result (first (sx-parse "#'(1 2 3)"))))
+      (assert-equal "quote" (symbol-name (first result)))
+      (assert-equal (list 1 2 3) (nth result 1)))))