Encode modified character keys through the kitty keyboard protocol - #583

Closes #579

Problem

When a TUI enables the kitty keyboard protocol, ghostel still encoded modified character keys with legacy sequences: M-<letter> went out as ESC <letter> (rejected as ambiguous by kitty-mode programs) and C-M-<letter> dropped meta entirely, sending the bare control byte. Modified functional keys (backspace, return, arrows) already encoded correctly.

Root cause

The native encoder built libghostty key events with only key and mods. libghostty resolves character keys through unshifted_codepoint and the generated text (utf8), so every modified character key produced no output at all — in both kitty and legacy modes — and fell through to the kitty-unaware elisp fallback ghostel--raw-key-sequence, whose ctrl branch also drops meta. Additionally, macos_option_as_alt defaulted to false on macOS builds, suppressing alt handling even with the codepoint present.

Fix

• input.keyEvent builder: single printable-ASCII keys carry their unshifted codepoint and the character as generated text; uppercase names fold to the lowercase codepoint with shift as a consumed modifier. Named/functional keys are untouched.
• macos_option_as_alt = .true: Emacs resolves option-vs-meta before the event reaches the module, so meta always means alt.
• C-<letter>, C-@, C-SPC, C-\ and ghostel-send-C-g now route through the encoder instead of raw-byte closures — legacy bytes are provably identical (libghostty's ctrlSeq yields the same C0 codes), kitty mode gets CSI-u.
• Raw C0 events 13/9/27 are remapped to return/tab/escape in ghostel--send-event (with the artifact ctrl modifier stripped): a TTY delivers RET as C-m, which the encoder would otherwise fixterms-encode as CSI-u and break Enter/Tab.
• Uppercase chords (delivered as lowercase base + shift) restore their case: M-T sends ESC T in legacy mode and reports the shifted alternate (\e[116:84;4u) under kitty flags=5.
• ghostel-send-C-c/-C-z/-C-d deliberately stay raw bytes so SIGINT/SIGTSTP/EOF reach the tty line discipline even while a hung program holds kitty mode.
• The elisp fallback's ctrl branch now ESC-prefixes when meta is set, so it no longer silently drops the modifier either.

API note: ghostel-send-key with "m"/"i"/"[" plus ctrl now yields the fixterms CSI-u sequence (matching upstream ghostty) instead of a C0 byte; live keystrokes are unaffected thanks to the C0 remap.

Testing

• 8 zig unit tests against the vendored encoder (kitty CSI-u, plain-text passthrough, legacy sequences, fixterms cases); src/input.zig is now emacs-free and part of zig build test.
• 6 native PTY byte-level ERT tests (kitty and legacy, meta/ctrl/ctrl-meta/shift chords) plus fallback, sim, and rescue-byte elisp tests.
• Live-verified in six sandboxed Emacs sessions with real keystrokes (TTY and GUI-shaped events): the issue's exact repro (printf '\e[>1u'; cat -v), TTY Enter/Tab/arrows, ESC-prefixed meta chords, readline editing, less/vi, and the rescue hatches interrupting a program that holds kitty mode.
• make -j8 all green.