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/sx/nav-data.sx

@@ -155,6 +155,8 @@
         :summary "Audit of all plans — what's done, what's in progress, and what remains.")
   (dict :label "Reader Macros" :href "/plans/reader-macros"
         :summary "Extensible parse-time transformations via # dispatch — datum comments, raw strings, and quote shorthand.")
+  (dict :label "Reader Macro Demo" :href "/plans/reader-macro-demo"
+        :summary "Live demo: #z3 translates SX spec declarations to SMT-LIB verification conditions.")
   (dict :label "SX-Activity" :href "/plans/sx-activity"
         :summary "A new web built on SX — executable content, shared components, parsers, and logic on IPFS, provenance on Bitcoin, all running within your own security context.")
   (dict :label "Predictive Prefetching" :href "/plans/predictive-prefetch"