Fix CSI Z (backtab / reverse tab) not being recognized as Shift+Tab. Previously, the Z final byte was not in the
functional key set, so terminals that emit ESC [ Z for Shift+Tab produced no usable input event. Now it correctly
maps to Tab with the Shift modifier.

Problem

Many terminals (xterm, GNOME Terminal, Windows Terminal in legacy mode) emit CSI Z when the user presses
Shift+Tab. Termina's CSI decoder did not recognize Z as a functional final byte, so backtab input was silently
dropped. This broke Shift+Tab navigation in pages that register it (e.g. clipboard gallery's focus cycling).

Changes

CsiFunctionalDecoder.cs — Add Z to IsFunctionalFinal, map it to ConsoleKey.Tab / TerminaKey.Tab, and attach the
Shift modifier via new FinalToModifiers / FinalToKeyModifiers helpers.

CsiFunctionalDecoderTests.cs — Three new tests: bare Z final returns Tab+Shift, same under Kitty-visible mode, and
IsFunctionalFinal('Z') returns true.

EscapeSequenceParserTests.cs — Integration test: full ESC [ Z sequence emits KeyPressed with ConsoleKey.Tab +
Shift.

Test plan

• dotnet test — all new and existing input parser tests pass
• Run the gallery demo, use Shift+Tab to cycle focus backward on the Clipboard page
• Verify regular Tab still cycles forward (no regression)