Modified character keys bypass the kitty keyboard protocol: M-<letter> sends legacy ESC, C-M-<letter> drops meta #579

Summary

When the running program has enabled the kitty keyboard protocol, ghostel
still encodes modified character keys with legacy sequences: M-<letter>
goes out as ESC <letter> (which kitty-mode programs reject as ambiguous
instead of reading as alt), and C-M-<letter> is sent as the bare control
byte — the meta modifier is silently dropped. Modified functional keys
(backspace, return, arrows) encode correctly as CSI-u.

In practice this makes alt-key bindings dead in semi-char mode for any
kitty-negotiating TUI (pi, Claude Code, kkp-enabled apps): M-b/M-f/M-d
word motion, C-M-<letter> chords, etc.

Steps to reproduce

1. M-x ghostel, then run: printf '\e[>1u'; cat -v (push the kitty
   disambiguate flag and capture input bytes).
2. Press M-t → cat shows ^[t. Expected under kitty: ^[[116;3u.
3. Press C-M-s → shows ^S (meta lost). Expected: ^[[115;7u.
4. Press M-<backspace> → shows ^[[127;3u — correct, demonstrating the
   functional-key path already does the right thing.

Same results via ghostel-send-key ("t" "meta" / "s" "ctrl,meta").

Root cause

The native encoder produces no output for these keys, so
ghostel--send-encoded falls back to ghostel--raw-key-sequence
(lisp/ghostel.el:1578). That function's character-key branches are
kitty-unaware: the ctrl branch matches first and emits char & 0x1f
(dropping any meta bit), and the meta+printable branch always emits
ESC <char>. Its special-key branches, by contrast, emit CSI-u for
modified variants — hence backspace/return/arrows working.

Suggested fix

When the terminal has kitty flags active, emit CSI-u for modified
character keys too (mirroring the existing special-key branches), keeping
the current legacy sequences for non-kitty programs.

Environment

• ghostel 0.47.0 (main, a2fe16d), prebuilt module
• Emacs 31.0.90 (emacs-plus, GUI), macOS aarch64-apple-darwin25.5
• Reproduced against latest main