fix: dedupe terminal pane focus + selection into engine method (#429)

[JDonaghy]

Summary

Both backends were inlining the same three steps after a content click on a non-split terminal pane:

```rust
engine.terminal_has_focus = true;
engine.terminal_scroll_reset();
if let Some(term) = engine.active_terminal_mut() {
term.selection = Some(TermSelection { ..click_cell });
}
```

Plus `handle_terminal_split_click` did the same work twice (once for `LeftPane`, once for `RightPane`) — its body literally repeated the three-step sequence after toggling `terminal_active`.

Extract `Engine::handle_terminal_pane_click(col, row)` as the single source of truth:

• TUI mouse handler consumes it in the non-split fallback (`mouse.rs:2069` → 5 lines collapsed to 1 engine call).
• `handle_terminal_split_click` LeftPane / RightPane arms delegate after setting `terminal_active`.

Net: `terminal_ops.rs` +3 / -3 (collapsed both arms), `tui_main/mouse.rs` -11 / -3.

Out of scope

GTK consumption (`src/gtk/mod.rs:6798-6811` has the same inline pattern) is a follow-up per the issue brief. Engine method is ready for GTK to consume in 1 line when that lands.

Test plan

• `cargo build`
• `cargo test --no-default-features --lib` — 1966 pass (no regressions). No new tests: the helper does 3 trivial state mutations and is covered transitively by the existing split-click smoke paths.
• `cargo fmt --check` on touched files
• TUI smoke (user-tested on server):
  • Click in non-split terminal — focuses, resets scroll, starts selection ✓
  • Click in left split pane — activates pane 0 + selection ✓
  • Click in right split pane — activates pane 1 + selection ✓
  • Found a pre-existing drag bug on right split (selection extends to wrong column) — not caused by this PR, filed as TUI right-split terminal: drag-select extends to wrong column (editor-relative vs pane-relative mismatch) #444. My PR preserves the existing click coord convention; the bug is in drag, which uses editor-relative col while click uses pane-relative.

Refs #429

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