fix: restore enhanced keyboard modes on PTY teardown

[inureyes]

Summary

Restore the local terminal's ordinary keyboard input mode after an interactive PTY session ends while a remote application has left enhanced keyboard reporting enabled.

What changed

• Centralized bracketed-paste, keyboard, mouse, alternate-screen, and cursor teardown in one best-effort writer used by both TerminalStateGuard::Drop and force_terminal_cleanup().
• Drain Kitty progressive-enhancement stacks and clear active flags before leaving the current screen, then repeat the reset on the main screen because the protocol maintains independent screen stacks.
• Disable xterm modifyOtherKeys alongside the Kitty protocol reset.
• Add exact-byte, idempotency, and writer-failure unit tests through the injectable writer.
• Document a Ghostty plus Kitty/WezTerm manual validation procedure and the current outer-TUI state-preservation limitation.

Test plan

• cargo test --lib pty::terminal::tests::
• cargo test --test pty_integration_test
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• git diff --check
• Repository pre-commit formatting and clippy hooks
• Manual real-TTY verification in Ghostty and a second Kitty-keyboard-compatible terminal

Known limitation

Cleanup restores the ordinary shell baseline rather than an enhanced keyboard mode owned by an outer local TUI. Crossterm 0.29 exposes support detection but not the active flag value; adding a custom /dev/tty query would introduce blocking and input-reader races and is intentionally not part of this teardown fix. Exact state preservation is tracked in #236.

Closes #234