Fix code block rendering: escape newlines/tabs in syntax highlighter output

highlight_sx/python/bash produced SX string literals with literal newline
and tab characters, breaking the wire format parser. Add centralized
_escape() helper that properly escapes \n, \t, \r (plus existing \\ and
" escaping). Code blocks now render with correct indentation and syntax
highlighting in both server and client renders.

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

sx/content/highlight.py

@@ -8,12 +8,22 @@ from __future__ import annotations
 import re
 
 
+def _escape(text: str) -> str:
+    """Escape a token for embedding in an SX string literal."""
+    return (text
+            .replace("\\", "\\\\")
+            .replace('"', '\\"')
+            .replace("\n", "\\n")
+            .replace("\t", "\\t")
+            .replace("\r", "\\r"))
+
+
 def highlight_sx(code: str) -> str:
     """Highlight s-expression source code as sx with Tailwind spans."""
     tokens = _tokenize_sx(code)
     parts = []
     for kind, text in tokens:
-        escaped = text.replace("\\", "\\\\").replace('"', '\\"')
+        escaped = _escape(text)
         if kind == "comment":
             parts.append(f'(span :class "text-stone-400 italic" "{escaped}")')
         elif kind == "string":
@@ -94,7 +104,7 @@ def highlight_python(code: str) -> str:
     tokens = _tokenize_python(code)
     parts = []
     for kind, text in tokens:
-        escaped = text.replace("\\", "\\\\").replace('"', '\\"')
+        escaped = _escape(text)
         if kind == "comment":
             parts.append(f'(span :class "text-stone-400 italic" "{escaped}")')
         elif kind == "string":
@@ -176,7 +186,7 @@ def highlight_bash(code: str) -> str:
     tokens = _tokenize_bash(code)
     parts = []
     for kind, text in tokens:
-        escaped = text.replace("\\", "\\\\").replace('"', '\\"')
+        escaped = _escape(text)
         if kind == "comment":
             parts.append(f'(span :class "text-stone-400 italic" "{escaped}")')
         elif kind == "string":