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>

TRANSCRIPT_RENDERING.md

@@ -239,6 +239,21 @@ and are the reason several tempting simplifications were rejected.
   to the end of the code. `HighlighterTest.kt` is the JVM unit test
   (`./gradlew :androidApp:testDebugUnitTest`); the cases in it are the
   library's mistakes, kept as regressions.
+  **Markdown is the one language that is not a `Rules` row**
+  (`MarkdownSyntax.kt`, added 2026-09-04). It has no keywords and no
+  strings, and what a character means there depends on where on the line it
+  sits -- a `#` opens a heading at the start of a line and is an ordinary
+  character three words in -- which is a question the token scanner cannot
+  ask. So it carries a scanner of its own, structure a line at a time and
+  then the inline forms left to right, and `spansOf` in `Languages.kt` is
+  the one entry point that hides which of the two a language got. It is
+  deliberately conservative where 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 in a paragraph opens nothing), an underscore may
+  not start or end inside a word (`snake_case_name`), and an indented code
+  block is left plain, because four spaces after a blank line and four
+  spaces after a bullet are the same line and only what came before tells
+  them apart.
   It replaced dev.snipme:highlights 1.1.0 on 2026-09-03, which found
   comments before it knew the language and paired `/*` with `*/` by
   ordinal. That library used one set of delimiters for every language, so