fix(keycodes): label Layer Lock keycode

[IgorArkhipov]

Summary

• show the QMK Layer Lock keycode as Layer Lock instead of raw 7C7B
• explain that the keycode locks or unlocks the highest active layer
• cover both the keycap label and tooltip with regression tests

Fixes #151

Verification

• cargo test --locked — 655 passed
• cargo clippy --locked --all-targets --message-format=short — passed with the existing warning baseline and no new warning in the changed code
• cargo build --locked
• git diff --check
• manual device validation was unavailable locally; the regression tests exercise the shared label and tooltip path used by Combo settings

Post-Deploy Monitoring & Validation

• during the next release smoke test, verify that a device reporting 0x7C7B shows Layer Lock and the matching tooltip instead of raw hex
• treat a raw 7C7B, an unknown-keycode tooltip, or an incorrect layer action description as a regression
• revert 7d9bdf3 if the QMK mapping proves incompatible with supported firmware