feat(editor): add .typoena.toml prefs and palette settings (v0.5 slice 4)

Git-tracked editor preferences read at boot and toggled live on-device:

- Prefs type (line-based TOML parse/serialize, no crate on xtensa) held on
  Editor; firmware reads /sd/repo/.typoena.toml before the first render and
  falls back to per-key defaults. Keys: save_on_idle, format_on_save,
  line_numbers (bool) + auto_sync (string, schema/default only until v0.7).
- line_numbers applied live (gutter_cols -> 0 when off).
- Palette > command mode toggles the three bools; the list stays open so
  several flip in one visit, and :settings opens it directly. Each toggle
  applies live and queues Effect::SavePrefs (host atomic-writes the file,
  which rides the next :sync).
- save_on_idle honoured host-side as a silent, unformatted idle auto-save.
- to_toml is newline-free; save_path now appends exactly one terminator
  unconditionally so buffers round-trip byte-for-byte (trailing blanks kept).
- Firmware 0.4.0 -> 0.5.0; new docs/typoena-toml.md reference. Also refreshes
  the slice-3 macroplan status (delete fix was confirmed on device).

firmware/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "firmware"
-version = "0.4.0"
+version = "0.5.0"
 authors = ["Julien Calixte <juliencalixte@gmail.com>"]
 edition = "2024"
 resolver = "2"