fix(control): preserve plain symbol key levels

[bvolpato]

Summary

Fix regression introduced by #128 where control-socket send-key retranslates plain shifted-level keysyms through level zero. For example, send-key at emits 2 instead of @.

Changes

• preserve requested keysym when Shift is absent
• use active GDK keymap translation only for explicit Shift
• retain Ctrl/Alt modifiers while preserving printable symbol text
• extend blocking live GTK smoke assertion from A! to @A!

Testing

export LD_LIBRARY_PATH="$PWD/ghostty/zig-out/lib:${LD_LIBRARY_PATH:-}"
./scripts/check.sh
LIMUX_SMOKE_PROFILE=debug ./scripts/xvfb-smoke-test.sh
shellcheck scripts/xvfb-smoke-test.sh
git diff --check

Results:

• 296 Rust tests passed
• 14 packaging checks passed
• live GTK bridge produced @A! through plain and shifted send-key calls
• independent deep review approved with no blocking findings

Did this cause any problems?

Revert this PR to restore #128 behavior.