v1.15.6

## Summary

- Hardened Engram Cloud startup repair against production schemas and historical invalid mutation rows.
- Prevented `engram cloud serve` restart loops during automatic mutation materialization.

## Details

- Legacy cloud chunk migration backfills now only reference legacy columns when they exist in the active schema.
- Mutation materialization repair now reports `invalid_mutations` and skips invalid historical rows instead of failing the whole repair.
- Added Postgres-backed regressions for modern production schemas without legacy columns and legacy session mutations missing required directories.

## Validation

- `go test ./internal/cloud/cloudstore`
- `CLOUDSTORE_TEST_DSN='postgres://engram:engram_dev@localhost:5433/engram_cloud?sslmode=disable' go test ./internal/cloud/cloudstore`
- `go test ./cmd/engram ./internal/cloud/cloudserver ./internal/cloud/remote ./internal/sync`
- `go test ./internal/server/...`
- `go test ./...`
- `git diff --check`
- Validated on the HostGator VPS before release: cloud health OK, dashboard HTTP 200, startup backfill completed, and post-repair dry-runs planned zero chunks.