Renumber commits as the triggering event, not a standalone message

Resolves the 01-vs-06 contradiction: the pure-diff composer cannot emit
'Renumber lane X' (order-only bookkeeping composes nothing), and a
renumber never happens alone — it batches with the insert/move that
exhausted the midpoint, so the commit reads as that event. 06's
bookkeeping rule now says sequence-preserving order changes are the
bookkeeping; repositioning order changes still compose Reorder.

Claude-Session: https://claude.ai/code/session_01HJ7PhFNmQ19bvy9RMD6GSb

DESIGN/06-history-undo.md

@@ -22,7 +22,7 @@ Git is the undo substrate — on boards that have git. **Git is opt-in per board
 
 The pathfinder's message engine carries over as the model — it is what earns the "semantic" in semantic commit messages, and it stays a pure, testable function:
 
-- **Pure snapshot diff, no write-site tagging.** Messages compose at commit time from a structural diff of two board snapshots (last-committed vs. current) — never by intercepting operations. Items match by id across the *whole* board, so a lane change is distinguishable from delete+add and a cross-lane move reads as a move. Bookkeeping — `order` renumbering, `modified`/`created` — produces no events: a diff touching only those composes nothing.
+- **Pure snapshot diff, no write-site tagging.** Messages compose at commit time from a structural diff of two board snapshots (last-committed vs. current) — never by intercepting operations. Items match by id across the *whole* board, so a lane change is distinguishable from delete+add and a cross-lane move reads as a move. Bookkeeping — `order` changes that preserve sibling sequence (a renumber's rescale — 01-storage-format.md), `modified`/`created` — produces no events: a diff touching only those composes nothing. Sequence is what the diff compares, not raw `order` values: an order change that *repositions* an item among its siblings composes Reorder, so a foreign writer's single-file reorder still reads as one. A midpoint-exhaustion renumber batches with the insert or move that triggered it, so its commit reads as that event.
 - **Vocabulary**: Add / Delete / Move / Rename / Edit / Restyle / Resize / Reorder over cards, lanes, and the board, plus Attach / Remove for attachment files ("Move card 'Fix login' to Doing", "Rename lane 'Todo' → 'Doing'"). One commit per window: a single event is the subject (with a detail body where one helps); several events of one kind fold into a plural subject, with shared destinations preserved ("Move 3 cards to Done"); genuinely mixed windows fall back to "Update board" — always with a bulleted body naming every event, so the oneline log stays scannable and the full message stays complete.
 - **Implied events don't steal the subject**: deleting a lane with five cards reads "Delete lane 'X'" with the card deletions as body bullets — not "Update board".
 - **Titles truncate in subjects only** (~40 chars, keeping `git log --oneline` sane); body lines carry full titles. An untitled item reads "(untitled)" — never a bare `""` (a pathfinder edge fixed, not carried). Undo/redo restores commit as "Undo: ⟨subject⟩" / "Redo: ⟨subject⟩"; the undo-menu labels are the *crossed* commit's subject, so labels never nest.