[Sync] Conflict resolution & merge policies

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Goal

Define and implement a deterministic conflict resolution strategy
for CoreData + CloudKit synchronization.

Context

When notes are modified on multiple devices (offline or online),
conflicts may occur during sync. This task ensures data consistency
and predictable merge behavior.

Scope

• CoreData merge policies
• CloudKit conflict handling
• Offline → online sync scenarios

Proposed Strategy

• Use last-write-wins as the baseline
• Prefer server-side changes when ambiguity exists
• Prepare the architecture for future custom merge logic

Acceptance Criteria

• No duplicate notes after sync
• Conflicts resolved deterministically
• Sync process does not block UI
• Behavior tested on at least two devices

Milestone

v1.1 – iCloud Sync