docs: complete gap analysis for all 7 sync engine problems

Analyzed and documented solutions for Jamie Turner's critical questions:
- Long Histories: State vector-based sync prevents data loss
- Schema Migrations: Two-phase approach (server + client)
- Protocol Evolution: Local storage migration on package updates
- Reset Handling: Synthesize CRDT deltas for manual edits
- Authorization: OUT OF SCOPE - developer responsibility

Status: 2/7 implemented, 4/7 fully planned, 1/7 out of scope

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: robelest