fix(backlog): post-merge review follow-ups (snapshot-merge union, canvas concurrency, 4 nits)

[New1Direction]

Summary

Clears the deferred findings from the stack pre-merge review. All on top of the merged stack; reviewed (APPROVE, 0 blocking) and unit-tested.

• importStores merge now unions snapshot history by timestamp (dedupe, re-clamp to cap) instead of overwriting — no data loss on backup restore. replace mode unchanged.
• renderCanvas mount-guard claimed synchronously + reset on failure — fixes a double-mount race between tab-switch and storage events.
• Canvas tab now refreshes from the CTA to a real Blueprint when Deep Dive finishes after the tab is open.
• 4 LOW nits: drop always-null snapshot version; no (+0) sparkline caption; count dropped invalid scenes into backup warnings; toCanvasSvg bounds via reduce (avoids RangeError on huge scenes).

Test plan

• 832 tests / 84 files green (+9 new regression tests, RED→GREEN)
• eslint 0 errors
• Adversarial review: APPROVE