Colour markdown, which is the one language that is not tokens

The token scanner asks what a character is; markdown's meaning is where it
sits, so a `#` opens a heading at the start of a line and is an ordinary
character three words in. `MarkdownSyntax.kt` reads structure a line at a
time and then each line's prose left to right, and `spansOf` is the one
entry point that hides which of the two scanners a language got.

Conservative wherever a guess would be invisible: emphasis needs a closer on
the same line with no space beside either marker, so the `*p = *q` of a C
fragment opens nothing; an underscore may not start or end inside a word;
and an indented code block is left plain, since four spaces after a blank
line and four after a bullet are the same line.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/androidApp/src/main/kotlin/com/example/aiapp/Highlighter.kt

@@ -60,7 +60,7 @@ data class SyntaxPalette(
  */
 fun highlight(code: String, language: Language?): AnnotatedString {
     if (language == null) return AnnotatedString(code)
-    val spans = DebugStats.timed("code highlighted") { scan(code, rulesOf(language)) }
+    val spans = DebugStats.timed("code highlighted") { spansOf(code, language) }
     val palette = catppuccinSyntax()
     return buildAnnotatedString {
         append(code)