Adopt quadraui TUI text-selection + OSC52 clipboard; delete bespoke clipboard/selection plumbing

[JDonaghy]

Blocked on

JDonaghy/quadraui#269 — tui runtime: line-wise mouse text selection + OSC52
copy via shared drag-origin dispatch. Don't start until that lands and the new
quadraui version is pinned here.

Why

quadraui #269 adds two things to the TUI runtime that let vimcode delete a stack
of bespoke, environment-fragile code:

1. OSC52 clipboard write in services().clipboard() — works over SSH/tmux,
   no external binary.
2. Cross-backend selectable text regions + DragTarget::TextSelection
   drag-origin dispatch, with line-wise selection and Ctrl-C copy.

Work

1. Delete the TUI clipboard plumbing (high confidence)

• Remove find_clipboard_write_cmd, find_clipboard_read_cmd,
  setup_tui_clipboard, tui_copy_to_clipboard (src/tui_main/mod.rs:644–771),
  plus the DISPLAY=:0 hack and the copypasta_ext-bug workaround.
• Route writes through services().clipboard().write_text() (OSC52 + arboard).
  Reads stay native/arboard — OSC52 read is disabled in most terminals.
• Reassess the engine.clipboard_read / engine.clipboard_write callback
  indirection — likely collapses into the quadraui service.
• sync_tui_clipboard (yank register → system clipboard) keeps its trigger but
  writes via the service.

2. Replace integrated-terminal screen selection (medium)

• The terminal panel's screen-cell selection + manual auto-copy-on-release
  (src/tui_main/mouse.rs:1145) maps onto quadraui's selectable-region +
  Ctrl-C/auto-copy. Register the terminal content rect as a TextRegion; drop the
  bespoke drag + auto-copy bookkeeping.

3. Route editor visual-selection drag arbitration (design needed)

• The editor's visual selection is document-model:
  window_zone_hit_test → WindowZone::TextArea { buf_line, col } → engine.mouse_drag(window, line, col) (src/tui_main/mouse.rs:1004). Keep the
  document mapping in the engine.
• Move the drag-origin arbitration + gesture lifecycle onto a quadraui
  DragTarget, mirroring how scrollbar drag already feeds through
  quadraui::dispatch_mouse_drag (src/tui_main/mouse.rs:51). Net: more bespoke
  mouse_text_drag bookkeeping in mouse.rs collapses.
• Open question to settle once Phase B.5e followup — quadraui::tui::run runner (after backend lift) #269 lands: whether DragTarget::TextSelection can
  carry a consumer payload (window/line/col) or vimcode needs a thin consumer-side
  variant.

Acceptance

• No xclip/xsel/wl-copy/pbcopy/clip.exe spawning in the vimcode TUI; copy works
  over SSH/tmux via OSC52.
• Mouse selection in editor + terminal panels goes through quadraui dispatch;
  src/tui_main/mouse.rs and src/tui_main/mod.rs shrink.
• Existing yank/paste and terminal copy behavior preserved.

Prereq: JDonaghy/quadraui#269

[JDonaghy]

Blocked on quadraui — do not start yet.

This depends on TUI runtime text-selection + OSC52 clipboard landing in quadraui first (per the platform-neutrality rule — no bespoke per-backend selection/clipboard plumbing in vimcode):

• JDonaghy/quadraui#269 — tui runtime: allow mouse text selection / copy of rendered content — OPEN
• JDonaghy/quadraui#283 — Lift mouse-drag text-selection + Ctrl-C copy from tui::run into shell_runner — OPEN

(For reference, GTK-side selection parity quadraui#272 has already landed/closed.) Once #269 + #283 land and the new quadraui is pinned here, the bespoke clipboard/selection code can be deleted.

[JDonaghy]

Coordinator: Assignment Complete

Machine: precision
Status: done
Exit code: 0
Duration: 2m 36s
Log: /home/john/.coord/logs/c5d195942fea.log

[JDonaghy]

Coordinator: Assignment Complete

Machine: dellserver
Status: done
Exit code: 0
Duration: —

Summary

Scoped #508 to Part 1 (per user's chosen scope): deleted bespoke xclip/xsel/wl-copy/wl-paste TUI clipboard plumbing (find_clipboard_write_cmd/find_clipboard_read_cmd/has_binary/setup_tui_clipboard's DISPLAY hack/StderrGuard), replaced with quadraui::tui::TuiPlatformServices (arboard + OSC52 + native-tool fallback), verified quadraui#269+#283 are merged and pinned. cargo build (both feature sets), clippy -D warnings (both), fmt, and full test suite all clean. Pushed commit 9a01c1f. Parts 2 (terminal-panel TextRegion migration) and 3 (editor visual-selection drag arbitration) deferred to follow-up issues per findings recorded in coord context on #508 — issue #508 itself should stay open until those follow-ups are filed/completed.