fix(research): stabilize shared history paging

[hubab1]

Summary

• replace mutation-prone offset/history cursors with bounded UTC-day and source-slot continuation
• preserve loaded observations when retrying a failed continuation page
• keep exact project and keyword generation validation inside the shared-store read

Validation

• 18 focused history and factory tests passed
• full suite: 453 tests in 42 suites passed
• signed macOS build succeeded and strict deep code-signature verification passed
• independent logic and UI-contract reviews found no remaining P1/P2 issues

Stack

• depends on feat(research): add keyword workspace actions #57
• kept draft because this is part of the persistence-backed research stack and should remain manually reviewed

No live-store application launch was performed.