feat: add BlockVol engine and iSCSI target (Phase 1 + Phase 2)
Phase 1: Extent-mapped block storage engine with WAL, crash recovery,
dirty map, flusher, and group commit. 174 tests, zero SeaweedFS imports.
Phase 2: Pure Go iSCSI target (RFC 7143) with PDU codec, login
negotiation, SendTargets discovery, 12 SCSI opcodes, Data-In/Out/R2T
sequencing, session management, and standalone iscsi-target binary.
164 tests. IQN->BlockDevice binding via DeviceLookup interface.
Total: 338 tests, 14.6K lines.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>