data_diff: add dedicated unit test file for pure functions

Flagged during v0.5.22 release review (Tech Lead persona).

The data_diff feature ships 1,164 lines in:
- `packages/opencode/src/altimate/native/connections/data-diff.ts`
- `packages/opencode/src/altimate/tools/data-diff.ts`

Coverage gap — no dedicated test file exercises the pure functions:
- `resolveTableSources`, `injectCte`
- `isAuditColumn`, `isAutoTimestampDefault`
- `buildColumnDiscoverySQL`, `parseColumnInfo`
- `quoteIdentForDialect`, `dateTruncExpr`
- `partitionMode`, `buildPartitionWhereClause`
- `mergeOutcomes`, `extractStats`

Create `packages/opencode/test/altimate/data-diff.test.ts` with table-driven tests per dialect (postgres, snowflake, bigquery, clickhouse, mysql, oracle, tsql). Estimated ~2 hours.

Deferred because: v0.5.22 ships adversarial tests covering the main attack surface via tool-facing paths. Dedicated unit coverage is a maintainability improvement, not a shipping blocker.