fix(obsidian): improve calendar event layout

[PANGKAIFENG]

Summary

• truncate long TaskNotes calendar event titles without hiding resize handles
• render concurrent events side by side with slotEventOverlap: false
• migrate the managed ATL unified calendar through Obsidian atomic processing while preserving user fields
• document the unified calendar behavior and prepare v0.5.6

Verification

• pnpm typecheck
• pnpm lint
• pnpm test (57 files, 594 tests)
• pnpm build
• git diff --check
• independent CR: no Critical or Important findings

Closes #56