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/Cargo.toml

@@ -88,6 +88,11 @@ log = "0.4"
 # Pure HID decode (Key type + edge-detecting Decoder), split out so it is
 # host-testable off the xtensa target. See ../keymap and MEMORY_AUDIT.md.
 keymap = { path = "../keymap" }
+# Editor core (buffer, motions, edits, layout/render) and the panel framebuffer,
+# extracted from src/editor.rs and src/epd.rs so `cargo test` can exercise them
+# off-device. The firmware links them; the host tests live in each crate.
+editor = { path = "../editor" }
+display = { path = "../display" }
 git2 = { version = "0.20", default-features = false, optional = true }
 esp-idf-svc = { version = "0.52.1", features = ["critical-section", "embassy-time-driver", "embassy-sync"] }
 # Remove `generic-queue-8` if you plan to use `embassy-time` WITH `embassy-executor`