docs(ui): classify App-resident runtime/derived fields (improvise-99k)

Step 4 of vb4. After steps 1–3, App owns model_state, view_state, and a
residue of five fields that don't belong to either slice. Add a top-level
doc on App explaining the slice structure and tag each residue field as
derived cache / runtime metadata / transient / config with a one-line
rationale. No behavior change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ui/app.rs

@@ -271,22 +271,50 @@ impl Default for ViewState {
     }
 }
 
+/// Top-level TUI application state. Composed of two named slices and a
+/// residue of fields that don't belong to either:
+///
+/// - `model_state` — document state (Workbook + IO bookkeeping). Persists.
+/// - `view_state`  — UI session state (mode, cursors, panels, buffers, …).
+///   Does not persist.
+/// - The remaining direct fields are runtime / derived / config — see
+///   per-field tags below. Each of them is documented as belonging to one
+///   of these residue categories so the slice boundary stays explicit:
+///
+///   - **derived cache**: a pure function of the slices, recomputed on
+///     demand. Owned by `App` because rebuilding belongs to the host.
+///   - **runtime metadata**: data the runtime feeds in (terminal dims,
+///     wall-clock instants). Not part of model or view state.
+///   - **transient**: a flag whose lifetime is bounded to one method call
+///     on `App`. Reset on entry, never observed across calls.
+///   - **config**: data loaded once at startup and read but not written
+///     during a session.
 pub struct App {
     pub model_state: ModelState,
     pub view_state: ViewState,
+    /// **Runtime metadata**: wall-clock instant of the last autosave. Used
+    /// by `autosave_if_needed` to debounce writes. Not part of either slice
+    /// because it's about IO timing, not document or session state.
     pub last_autosave: Instant,
-    /// Terminal dimensions (updated on resize and at startup).
+    /// **Runtime metadata**: terminal dimensions, updated on resize events
+    /// and at startup. Fed in by the host loop in `draw.rs`.
     pub term_width: u16,
     pub term_height: u16,
-    /// Current grid layout, derived from model + view + drill_state.
+    /// **Derived cache**: pure function of `model_state.workbook`,
-    /// Rebuilt via `rebuild_layout()` after state changes.
+    /// `view_state.drill_state`, and the active view. Rebuilt via
+    /// `rebuild_layout()` after any state change. Owned by `App` because
+    /// it's the host's job to coordinate the rebuild, not the slices'.
     pub layout: GridLayout,
-    /// When set to true by an effect during `apply_effects`, the remaining
+    /// **Transient**: when an effect sets this to `true` during
-    /// effects in the batch are skipped. The flag is reset at the start of
+    /// `apply_effects`, the remaining effects in the batch are skipped.
-    /// every `apply_effects` call. Use via the `AbortChain` effect — this is
+    /// Reset to `false` at the start of every `apply_effects` call. Use
-    /// the mechanism by which e.g. "advance at bottom-right" short-circuits
+    /// via the `AbortChain` effect — this is the mechanism by which e.g.
-    /// the trailing `EnterEditAtCursor` in a `CommitAndAdvance` chain.
+    /// "advance at bottom-right" short-circuits the trailing
+    /// `EnterEditAtCursor` in a `CommitAndAdvance` chain.
     pub abort_effects: bool,
+    /// **Config**: keymap configuration assembled at startup by
+    /// `KeymapSet::default_keymaps()`. Read every keypress but never
+    /// mutated during a session.
     keymap_set: KeymapSet,
 }