Verifies the user-visible history contract the dispatcher owes:
1. One batch dispatch = exactly ONE undo entry (even for 8 tools).
No Ctrl-Z spam to reverse one AI turn.
2. Separate dispatches = separate undo entries. The batch window
closes at endBatch; subsequent dispatches don't piggyback.
3. Undo reverts the whole batch atomically (all tools snap back).
4. Undo → redo restores the whole batch atomically.
5. All-unsupported batch creates ZERO undo entries — endBatch's
"no changes" short-circuit prevents ghost undos that would jump
the UI between identical states.
6. Mixed valid+unsupported batch → 1 undo entry for the valid ones.
Complements element-tools-dispatcher.test.ts (which spies on
startBatch/endBatch) by exercising the actual history-store round-
trip and asserting stack length deltas.
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