Add an in-TUI editor, solution diff, and lesson-first flow

[cheesecakeMafia]

Why

Learning Rust in this tool meant keeping two terminals open: the game in one,
your editor in the other. That split is where the friction was — every fix
needed a window change, and the compiler output you were fixing lived in the
window you had just left.

It also fixes the problem that started the project. Hitting println without
the ! on exercise one and having nowhere to turn is exactly what the lesson
gating is for: the module's lesson now arrives before its exercises rather than
sitting behind a marker you can ignore.

What

Three commits, ordered so each builds and tests on its own.

d48514d — fix: fire a save that lands inside the debounce window

A pre-existing bug found while building the rest. A second save arriving within
300ms of another was recorded and then dropped: the block meant to fire it later
had an empty body. Its regression test fails against the old logic and passes
against the new one.

9bb2fc4 — feat: track layout and viewed solutions in state v3

Schema bump for the two new pieces of persisted state. Both fields carry
#[serde(default)] so a v2 file parses before migration runs. The migration
test asserts every prior exercise, module and lesson entry survives — a schema
bump that quietly drops progress is indistinguishable from a corrupt file.

ba2f80c — feat: add an in-TUI editor with lesson gating and diff

Watch mode becomes an editable split. Ctrl+S saves and the existing watcher
drives verification. Ctrl+D shows your code beside the reference solution with
differing lines highlighted, recorded but never penalised. Ctrl+J switches
between side-by-side and stacked. An unread lesson opens before its exercises;
Esc skips it for the session.

Things worth knowing when reading the diff

• edtui's EditorState defaults to Normal mode, where its modeless keymap
  binds nothing. Both construction sites set Insert explicitly — without it
  the editor renders perfectly and ignores every keystroke.
• Saves are atomic (temp file plus rename). A partial write would fire the
  watcher and, under disk-wins, reload the truncated file over the live buffer.
• Our own writes are fingerprinted: they still trigger verification but never a
  reload, which would discard anything typed while the compile ran.
• Ctrl+I/Ctrl+M arrive as Tab and Enter, Ctrl+H is how some terminals send
  Backspace, and Ctrl+Q/T/W/Z are claimed by the terminal emulator. All are
  left unbound — a binding taken upstream never reaches the application.
• Search moved to Ctrl+F because Ctrl+S is now save. edtui registers
  Ctrl+S in two modes, so both had to move or repeat-search would silently
  stop working.
• The editor pane refuses to render below a size floor: edtui's line wrapper
  spins forever at zero content width, and a hang defeats the panic hook that
  would otherwise restore the terminal.

Verification

• cargo clippy --all-targets -- -D warnings — clean
• cargo test — 182 passing, 0 failing (was 150)
• cargo test --test solution_verification_test -- --ignored — 64/64 solutions
• Commits 1 and 2 verified building and testing standalone in a scratch copy