feat(keymap): extract pure HID decode into host-testable crate

The Key type, US-QWERTY translate, and the edge-detecting boot-report
parser lived in usb_kbd.rs, unreachable by cargo test (the firmware
crate is pinned to the xtensa target). Move them to a dependency-free
#![no_std] + #![forbid(unsafe_code)] crate so the one path that parses
untrusted device bytes can be exercised on the host.

14 tests, including an ASCII-invariant sweep over all 256 usage IDs
(pins the guarantee the editor's byte==char indexing relies on) and a
never-panics fuzz over arbitrary-length/content reports.

keymap/.gitignore

@@ -0,0 +1,2 @@
+/target
+/Cargo.lock