Step whole characters everywhere instead of assuming 1 byte = 1 char, so
the dead-key composer's accented Latin-9 output (é, ç, …) no longer traps
the caret mid-character and panics on the next edit. Covers h/l/a and the
Escape step-back, j/k column math, backspace and x, word-end (e) with
de/ce, and layout/caret_rc (byte offsets vs display columns). Adds 7
accented-input tests (15 total).
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.
