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/FileLines.kt

@@ -73,8 +73,7 @@ private constructor(
             DebugStats.timed("file scanned and cut into lines") {
                 val body = text.removeSuffix("\n")
                 val lines = body.split('\n')
-                val rules = language?.let { rulesOf(it) }
-                val scanned = if (rules == null) emptyList() else scan(body, rules)
+                val scanned = if (language == null) emptyList() else spansOf(body, language)
                 FileLines(lines, bucket(lines, scanned), lines.maxOf(::columnsOf))
             }