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/static/scripts/sx.js

@@ -70,6 +70,9 @@
   function isMacro(x) { return x && x._macro === true; }
   function isRaw(x) { return x && x._raw === true; }
 
+  // --- Reader macro registry ---
+  var _readerMacros = {};
+
   // --- Parser ---
 
   var RE_WS = /\s+/y;
@@ -155,6 +158,9 @@
       }
     }
 
+    // Reader macro dispatch: #
+    if (ch === "#") { this._advance(1); return "#"; }
+
     // Symbol
     RE_SYMBOL.lastIndex = this.pos;
     m = RE_SYMBOL.exec(this.text);
@@ -171,6 +177,27 @@
     throw parseErr("Unexpected character: " + ch + " | context: «" + ctx.replace(/\n/g, "\\n") + "»", this);
   };
 
+  Tokenizer.prototype._readRawString = function () {
+    var buf = [];
+    while (this.pos < this.text.length) {
+      var ch = this.text[this.pos];
+      if (ch === "|") { this._advance(1); return buf.join(""); }
+      buf.push(ch);
+      this._advance(1);
+    }
+    throw parseErr("Unterminated raw string", this);
+  };
+
+  Tokenizer.prototype._readIdent = function () {
+    RE_SYMBOL.lastIndex = this.pos;
+    var m = RE_SYMBOL.exec(this.text);
+    if (m && m.index === this.pos) {
+      this._advance(m[0].length);
+      return m[0];
+    }
+    throw parseErr("Expected identifier after #", this);
+  };
+
   function isDigit(c) { return c >= "0" && c <= "9"; }
 
   function parseErr(msg, tok) {
@@ -199,6 +226,33 @@
       }
       return [new Symbol("unquote"), parseExpr(tok)];
     }
+    // Reader macro dispatch: #
+    if (raw === "#") {
+      tok._advance(1); // consume #
+      if (tok.pos >= tok.text.length) throw parseErr("Unexpected end of input after #", tok);
+      var dispatch = tok.text[tok.pos];
+      if (dispatch === ";") {
+        tok._advance(1);
+        parseExpr(tok); // read and discard
+        return parseExpr(tok); // return next
+      }
+      if (dispatch === "|") {
+        tok._advance(1);
+        return tok._readRawString();
+      }
+      if (dispatch === "'") {
+        tok._advance(1);
+        return [new Symbol("quote"), parseExpr(tok)];
+      }
+      // Extensible dispatch: #name expr
+      if (/[a-zA-Z_~]/.test(dispatch)) {
+        var macroName = tok._readIdent();
+        var handler = _readerMacros[macroName];
+        if (!handler) throw parseErr("Unknown reader macro: #" + macroName, tok);
+        return handler(parseExpr(tok));
+      }
+      throw parseErr("Unknown reader macro: #" + dispatch, tok);
+    }
     return tok.next();
   }
 
@@ -1500,6 +1554,9 @@
       }
     },
 
+    /** Register a reader macro: Sx.registerReaderMacro("z3", fn) */
+    registerReaderMacro: function (name, handler) { _readerMacros[name] = handler; },
+
     // For testing / sx-test.js
     _types: { NIL: NIL, Symbol: Symbol, Keyword: Keyword, Lambda: Lambda, Component: Component, RawHTML: RawHTML },
     _eval: sxEval,