refactor(editor): extract editor + display into host-testable crates

Move the editor core out of the firmware crate (pinned to the xtensa
target, so it can't run `cargo test`) into a standalone `editor` crate,
with the panel framebuffer + geometry split into a `display` crate. Both
depend only on embedded-graphics + keymap, so the editor is now
host-buildable and unit-tested (8 characterization tests). Firmware links
them and re-exports the geometry from epd.rs; behaviour is unchanged.

The xtensa firmware build was not verified in this environment.

firmware/src/main.rs

@@ -1,4 +1,3 @@
-mod editor;
 mod epd;
 mod usb_kbd;