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).

docs/README.md

@@ -15,6 +15,7 @@
 | [`v0.1-mvp-product.md`](v0.1-mvp-product.md) | v0.1 product design — boot, type one file, `Ctrl-S` to save, `Ctrl-G` to publish. |
 | [`v0.1-mvp-technical.md`](v0.1-mvp-technical.md) | v0.1 technical design — single Rust binary on `esp-idf-rs`, modules, threads, bring-up order. |
 | [`macroplan.md`](macroplan.md) | Version-by-version plan; each release is a usable artifact, not a checkpoint. |
+| [`typoena-toml.md`](typoena-toml.md) | `.typoena.toml` reference — the git-tracked editor preferences (auto-save, format-on-save, line numbers, auto-sync). |
 | [`hardware.md`](hardware.md) | Part choices for the bench build and the rationale behind them. |
 
 ## Quality method