TUI: mouse drag selection with OSC 52 copy; Markdown emphasis follows CommonMark; v0.5.0

- Drag, double-click (word) and triple-click (line) selection in the log,
  auto-scroll at the edges, soft-wrapped lines joined on copy.
- sashidown ANSI renderer: delimiter runs with flanking rules, so
  intraword underscores (test_apply_patch.c) stay literal; unmatched
  * _ and backticks are shown as-is; * and _ do not close each other.
- tests: test_textsel, test_markdown.

docs/compat.md

@@ -81,9 +81,15 @@ take the agent down.
 - UTF-8 locale gives the full UI. Under a non-UTF-8 locale, or with
   `CST_ASCII=1`, box and icon glyphs fall back to ASCII.
 - `NO_COLOR=1` turns colours off.
-- `/copy` uses OSC 52. iTerm2 needs "Applications in terminal may access
-  clipboard"; tmux needs `set -g set-clipboard on`; Windows Terminal (WSL)
-  supports it out of the box.
+- `/copy` and mouse selection use OSC 52. iTerm2 needs "Applications in
+  terminal may access clipboard"; tmux needs `set -g set-clipboard on`;
+  Windows Terminal (WSL), kitty, WezTerm, foot and WHC Shell support it out of
+  the box.
+- Mouse selection: the TUI turns on button-motion reporting (`?1002h`) on top
+  of what ncurses enables, so dragging over the transcript selects text
+  (double-click a word, triple-click a line) and releasing copies it. Terminals
+  without 1002 still get clicks and the wheel. Shift+drag bypasses the TUI and
+  uses the terminal's own selection.
 - Bracketed paste needs ncurses (all platforms above).
 - macOS Terminal.app sends Option+Enter as Esc+Enter only with "Use Option as
   Meta key"; Ctrl+N always inserts a newline.
@@ -128,6 +134,9 @@ TUI:
 - [ ] `/diff`, `/export`, `/status`, `/tools off grep`, `/yolo` work
 - [ ] Mouse: click a tool card to expand it, click in the input to move the
       cursor, click a picker row; `/mouse` turns it off
+- [ ] Drag over a wrapped reply: it highlights, the status row says "Copied",
+      and pasting gives the paragraph without extra line breaks; double-click
+      copies a word
 - [ ] Tab toggles Build/Plan, Shift+Tab cycles ask/auto/yolo, Ctrl+P opens the palette
 - [ ] `/model` lists the provider's `/models` (marked `(api)`); a wrong key is re-asked
 - [ ] `auto` mode: `echo x > ~/outside` in run_shell asks "Rerun outside the