Add integration tests validating Dolt APIs needed for peer-to-peer
federation between Gas Towns:
- TestFederationDatabaseIsolation: verify separate towns have isolated DBs
- TestFederationVersionControlAPIs: branch, checkout, merge, commit
- TestFederationRemoteConfiguration: AddRemote accepts dolthub:// URLs
- TestFederationHistoryQueries: commit history, dolt_log, AS OF
Key finding: Production federation uses dolt sql-server remotesapi
(port 8080) for direct peer-to-peer push/pull - no DoltHub required.
Also fixes concurrent_test.go Merge() return value handling.
Refs: bd-wkumz (Federation epic), hq-ew1mbr.7 (prototype task)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
4f3c3febd5