fix(tui): the title bar's escape hatch outranks the document name (improvise-t14d)

`fill_line` composed the title bar's two segments by concatenating them and
letting ratatui clip. ratatui clips the RIGHT, and the right segment is
" ?:help  :q quit " -- the only thing on screen telling a stuck user how to get
help or get out. The left segment is decoration whose length is unbounded, so
the affordance was the first thing sacrificed: measured through a TestBackend, a
document named "Quarterly Regional Sales Analysis" stored as
quarterly-regional-sales-2026.improv loses ":q quit" at 100 columns.

Two smaller faults in the same three lines. The padding was `left.len()` --
BYTES -- and the title's own format string spells a "·", two bytes and one cell,
so every document's right segment sat one cell in from the edge it is anchored
to, before any non-ASCII name made it worse. And there was no truncation at all,
so the failure was a mid-token clip of "?:" rather than a graceful drop.

`Keeps` makes the priority a per-call-site decision rather than a global rule,
because it genuinely differs: the title bar keeps the RIGHT (the hatch outranks
the name), while `minibuffer_line` and `draw_status` keep the LEFT, which is what
they already did -- there the left is the user's own typing plus the cursor, or
the status message, and right priority would let a long message eat the text
being edited. Truncation goes through ui::text::truncate, so widths are display
cells throughout and an overflow is an ellipsis.

Five tests, each rendered through a real TestBackend because the loss happens in
ratatui's clip and a helper's return value cannot see it. Three faults injected,
each caught by a distinct test; the reasoning and the injections are in
context/test-evidence/tui.md and context/code/tui.md.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH

context/code/tui.md

@@ -154,6 +154,35 @@ composes both: the prompt and buffer on the left, any pending `status_msg`
 right-aligned. The alternatives and why they lost are in
 `context/test-evidence/tui.md`.
 
+### `fill_line` — which segment yields is a per-call-site decision
+
+Three rows in the TUI are one left segment and one right segment sharing a line:
+the title bar, `minibuffer_line` and `draw_status`. `fill_line` composes them,
+and it owes two things that it did not originally do — it must measure in
+**display cells** (it padded with `left.len()`, bytes, and the title's own `·`
+is two of them, so every document's right segment sat one cell in from its edge)
+and it must **truncate**, because concatenating and letting ratatui clip clips
+the *right*.
+
+What clipping the right costs depends on the row, which is why `Keeps` is a
+parameter and not a rule:
+
+- **The title bar keeps the right.** Its left segment is decoration whose length
+  is unbounded — a document name plus a filename — and its right segment is the
+  fixed `" ?:help  :q quit "`, the only thing on screen telling a stuck user how
+  to get out. Unconditional left priority made the escape hatch the first thing
+  sacrificed, at 100 columns with an ordinary document name (improvise-t14d).
+- **Both bottom-bar rows keep the left**, which is what they already did. There
+  the left is the user's own typing plus the cursor (`minibuffer_line`) or the
+  status message (`draw_status`), and the right is a view badge or the message's
+  own margin. Right priority there would let a long message eat the text being
+  edited — the same mistake in the other direction.
+
+So the rule is not "the right wins" but "the affordance wins over the
+decoration", and which is which is local. Truncation goes through
+`ui::text::truncate` (improvise-8l0c), so an overflow is a `…` rather than a
+mid-token clip of `?:`.
+
 ### The caches are built here, per frame
 
 `project::formulas_of`, `category_tree_of`, `view_list_of`, `tile_bar_of`,