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>

sx/sxc/pages/docs.sx

@@ -629,6 +629,7 @@
   :content (case slug
     "status" (~plan-status-content)
     "reader-macros" (~plan-reader-macros-content)
+    "reader-macro-demo" (~plan-reader-macro-demo-content)
     "sx-activity" (~plan-sx-activity-content)
     "predictive-prefetch" (~plan-predictive-prefetch-content)
     "content-addressed-components" (~plan-content-addressed-components-content)