feat: add mouse selection and copy/paste to embedded terminal

[Wirasm]

Summary

• Wire GPUI mouse events (MouseDown, MouseMove) to alacritty_terminal's Selection API for click-drag text selection
• Render selection as semi-transparent ice-colored highlight quads between cell backgrounds and text
• Cmd+C copies selection to clipboard, sends SIGINT (0x03) when no selection active
• Cmd+V pastes clipboard contents to PTY stdin
• Support single-click (simple), double-click (word), and triple-click (line) selection

Closes #313

Test plan

• Click-drag selects text with visual highlight
• Double-click selects word, triple-click selects line
• Cmd+C with selection copies to clipboard (verify paste in another app)
• Cmd+C without selection sends SIGINT (Ctrl+C behavior preserved)
• Cmd+V pastes clipboard content into terminal
• Click clears existing selection
• Selection renders behind text (text remains readable)

[Wirasm]

PR Review Summary

Critical Issues (0 found)

None.

Important Issues (3 found — all addressed)

Agent	Issue	Location	Status
comment-analyzer	Mouse down handler comment says "clears selection" but code always creates new	terminal_element.rs:677	Fixed
comment-analyzer	Cmd+C comment missing selection-clearing behavior	terminal_view.rs:80	Fixed
code-reviewer	pixel_to_grid missing bounds clamping before as i32/as usize casts	terminal_element.rs:110	Fixed

Suggestions (4 found — triaged, not actioned)

Agent	Suggestion	Location	Decision
silent-failure-hunter	PTY write failures (Cmd+C/V) have no user-visible feedback	terminal_view.rs:87-99	Skip — already logged with tracing::error; batch loop catches dead PTY and sets error_state
silent-failure-hunter	write_to_clipboard has no error path, selection cleared regardless	terminal_view.rs:84	Skip — GPUI API is infallible by design
silent-failure-hunter	Selection rendering silently clamps out-of-bounds lines	terminal_element.rs:451-475	Skip — defensive clamping is correct; debug logging for alacritty internals is over-engineering
comment-analyzer	pixel_to_grid doc comment could explain Side calculation	terminal_element.rs:95	Added clamping note; Side logic is self-evident from code

Strengths

• Clean layered rendering architecture (Layers 1–5) with selection fitting naturally at 2.5
• Proper structured logging following project conventions (ui.terminal.*)
• Smart Cmd+C dual behavior (copy vs SIGINT) matches native terminal UX
• Selection types (simple/word/line via click count) are clean and correct

Verdict

READY TO MERGE after pushing the 3 fixes above.