feat: preserve outer TUI keyboard state

[inureyes]

Summary

• Capture raw Kitty enhancement flags independently for the main and alternate screens, together with xterm modifyOtherKeys and DEC private mode 1049, in DA-delimited transactions bounded to 100 ms per screen before the PTY input reader starts.
• Preserve every post-DA, unrelated, malformed, partial, or concurrently typed byte in order and forward it through the existing local escape detector and PTY input path.
• Normalize both screens to a legacy keyboard baseline for the remote session without popping the outer application's Kitty stacks, then reset remote leaks and restore both captured screen states on normal, forced, and repeated cleanup paths.
• Track only successfully delivered, filtered 7-bit DEC 1049 transitions with an OSC/DCS/APC/PM/SOS-aware streaming parser. Cleanup safely leaves a remote-owned 1049 screen when bssh began on main, while avoiding destructive 1049h when bssh began inside an outer alternate-screen TUI.
• Reject concurrent raw and deferred-raw terminal guards with an atomic RAII owner so process-global cleanup snapshots cannot overwrite each other, while retaining the fix: restore enhanced keyboard modes after PTY disconnect #234 ordinary-shell fallback when state capture is unsupported or incomplete.

Test plan

• cargo test --lib (1,280 passed, 9 ignored)
• cargo test --test pty_integration_test (18 passed)
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• Manual outer-TUI verification in Ghostty and Kitty or WezTerm (procedure documented; unavailable in the automated execution environment)

Closes #236