tui: render what the model says as the Markdown it is

The transcript has always word-wrapped assistant text and left it at
that, so a table arrived as a wall of pipes and a heading as a line
beginning with a hash. Everything the model writes is Markdown; the one
part of the screen that had to know that was the part that did not.

Assistant text now goes through glamour, with a stylesheet built from
the theme in force rather than glamour's own palette, so headings take
the accent, code blocks are chroma-highlighted in the same colours the
rest of the UI uses, and /setup repaints Markdown along with everything
else. Headings drop their hashes: having obeyed the instruction there is
no reason to print it too, and the levels are told apart by colour and
weight, the way this UI tells everything else apart. It renders as it
streams, so a half-typed fence is a code block in progress.

Only what the model says goes through it. Tool results and the output of
your own ! and $ commands are program output and are shown as they came,
and -no-tui is untouched so -p stays pipeable.

Four things the library needed working around. Styles.Text is applied as
an override on every text run, so a colour there flattens headings and
links back to one colour — Document carries the base instead. glamour
registers its chroma palette under one global name and skips the work if
taken, which would freeze code blocks in whichever theme drew first, so
we register our own per theme. Its default formatter quantises to 256
colours, a shade off everything beside it. And it re-emits the same
colour per word and one escape per cell of padding — an order of
magnitude more escape than text, held for the session — so each line is
compacted to the escapes it actually needs.

A word too long to wrap is now broken by force rather than left hanging
over the edge for the viewport to wrap again at a column this layout
knows nothing about. Laying out every entry on every streamed token cost
32ms on a fifty-message session, so an entry keeps its rendered
Markdown; the theme generation counter that drops it also fixes the
scrollback keeping its old colours after a theme change, which it did
before any of this.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>