Persist the custom dictionary across sessions (#40) (#796)

* Persist the custom dictionary across sessions (#40)

* Review fixes for #796: session-only Ignore All, desktop-only persistence, capped atomic I/O

- review.ignoreAll no longer adds to (and saves) the custom dictionary: ignored words go to a
  separate session-only list, as Ignore All does in Word.
- Only the desktop app wires the dictionary file. Headless MCP and `wordcraft-cli run` keep
  words in memory, so an agent never reads or changes the person's dictionary (and nothing
  writes outside an automation write root, #769).
- Loading reads through `take(1 MB)`, so a huge file is never read whole; lines that aren't a
  single word (spaces, control characters, invalid UTF-8, over 64 characters) are skipped.
- review.addToDictionary applies the same word rule (proof::custom_word), so a word added is
  never dropped on the next load.
- Saving merges the words on disk first (another process's additions survive; words just
  removed stay out), creates the folder, and replaces the file atomically (temp + rename).
  The file I/O lives in the proof crate, so the engine doesn't touch std::fs itself.
- Tests for each.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Brandon Thomas <echelon@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.lock

@@ -5020,6 +5020,7 @@ dependencies = [
  "wordcraft-doc",
  "wordcraft-engine",
  "wordcraft-fonts",
+ "wordcraft-proof",
  "wordcraft-ui-egui",
  "zbus",
 ]