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/ui-sandbox.sh

@@ -270,6 +270,27 @@ printf 'def main():\n    # a comment\n    print("hello")\n' >"$FILES/main.py"
 printf '#!/bin/sh\n# a comment\necho hello\n' >"$FILES/run.sh"
 chmod +x "$FILES/run.sh"
 printf '{"a": 1, "b": [true, null]}\n' >"$FILES/data.json"
+# Markdown's scanner is line-structured rather than tokens, so the fixture holds one of each
+# thing it decides by position: a heading, a fence, a list, a quote, a link and a rule.
+cat >"$FILES/notes.md" <<'MARKDOWN'
+# Notes
+
+A paragraph with `code`, **bold** and a [link](PLAN.md).
+Not emphasis: a * b * c, and snake_case_name.
+
+## A list
+
+- one
+- two
+
+> quoted
+
+```rust
+fn main() { println!("hello"); }
+```
+
+---
+MARKDOWN
 # Not UTF-8, so it reads as binary rather than as mojibake.
 printf '\377\376\000\001binary\n' >"$FILES/picture.bin"
 # Over FILE_LIMIT (1 MiB), so the read refuses before anything transfers.