Colour markdown's tables and the addresses written in it

A table is recognised by its delimiter row, the only line of one that cannot
be anything else, and its header is the line above -- the single place the
scanner looks ahead. Colouring every `|` instead would have marked the pipes
of a shell command written in a paragraph.

Addresses come in two shapes: `<...>` needs a scheme's colon or an at sign
inside it and no whitespace, which leaves `<div>` alone; a bare `scheme://`
needs no closer, so where it ends is the decision -- the sentence's trailing
punctuation is given back, and so is a closing bracket unless one opened
inside the URL.

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

app/ui-sandbox.sh

@@ -285,6 +285,13 @@ Not emphasis: a * b * c, and snake_case_name.
 
 > quoted
 
+| column | what it holds |
+|--------|---------------|
+| one    | a value       |
+
+A link <https://example.com> and a bare https://example.com/a., but run a | b
+in a paragraph has no table in it.
+
 ```rust
 fn main() { println!("hello"); }
 ```