Placeholder: composer undo follow-up - #11

Nested key handlers can now tell the outer dispatcher when they already
changed undo history, and retained-byte accounting fails loudly if it drifts.

- Rename the internal revision to `mutation_epoch` and advance it for every
  record, clear, undo, and redo. An unchanged epoch leaves the outer
  dispatcher responsible for recording the edit.
- Route pops, limit eviction, and stack clearing through checked byte removal
  instead of saturating subtraction, so corruption cannot silently weaken
  the memory cap.
- Derive fallback conflict detection from typed default undo and redo bindings
  while retaining exact terminal compatibility aliases.
- Cover duplicate outer recording after a nested history mutation and
  retained-byte underflow.

Shell-style Up/Down now reuses an adjacent undo or redo state when the
recalled draft has the same complete content. Unrelated prompts still
establish a new baseline, so navigation itself never becomes an undo step.

- Carry Older/Newer through HistoryNavigation so cached and asynchronous
  recall share one transition. Rename HistorySearchDirection because it
  already serves normal navigation.
- Compare complete EditableDraftContent but restore the exact stored
  snapshot, preserving cursor and rich state without a second buffer.
- Cover Ctrl+C -> Up -> Ctrl+Z, symmetric Down/redo, unrelated and
  equal-text rich-state mismatches, and asynchronous lookup.

Vim commands and ordinary composer edits now share one bounded undo/redo lineage, so undo can cross mode switches without maintaining two stacks.

- Keep Vim state as a pending transaction coordinator and commit complete commands or insert sessions into ComposerUndoHistory.
- Route Vim u and composer undo/redo through the same rich-draft restoration path while preserving repeat and search state.
- Suppress ordinary per-key records when Vim completion owns the action boundary, preventing duplicate history entries.
- Cover mixed-mode traversal, mode switches, and active insert-session undo alongside the upstream Vim behavior.

Retro-captured paste bursts no longer leave per-character undo records that
can resurrect only part of the paste.

- Remove the detector-reported number of provisional edit records when an
  existing retro-capture path reclassifies typed characters as one paste,
  then let the normal paste flush record the final action once.
- Advance the history mutation epoch during removal so the outer dispatcher
  does not recreate a discarded entry.
- Keep paste thresholds, timing, and terminal support unchanged.
- Cover whole-paste undo and redo through the existing detector.

Retro-capture now validates the exact undo suffix before removing provisional
entries, so a stale detector count cannot consume older history.

- Pair the detector count with the complete pre-capture `EditableDraft` and
  require the first candidate suffix entry to match it. A missing or stale
  boundary leaves history unchanged.
- Centralize textarea removal, history validation, and paste-buffer seeding in
  one transition shared by the ASCII and non-ASCII capture paths.
- Advance the mutation epoch only after validated removal, keeping the outer
  dispatch transaction coherent.
- Cover invalid boundaries, preservation of an older undo entry, non-ASCII
  retro-capture, and a second undo that cannot resurrect the paste prefix.

Paste-burst grouping now lands atomically in PR #10 because the core undo PR
must be correct on its own. This empty commit keeps the stacked branch and PR
available for later follow-up work without carrying a second implementation.