[belikh]

Refs #336 (approved direction), #265, #279, #283.

What this changes

src/scsi.rs's DiskBackend enum is replaced by BlockNode in src/block_node.rs, with one implementation per backend:

backend	node
raw file / ISO	RawNode
raw .overlay COW	CowNode
hard-disk CHD	ChdNode
CD CHD	ChdCdNode

The trait is byte-addressed (size, read, write, flush, discard, block_status, backing) and carries the divergence contract (dirty_count, is_cow, commit, rollback, overlay_paths, pending_sync). ScsiDevice holds Option<Box<dyn BlockNode>>; perform_read/perform_write translate LBA to bytes once. CowNode::write does the 512-byte read-modify-write for a partial sector; a raw discard zero-fills, a COW discard releases the range back to the backing. block_status describes a homogeneous run, so a caller can step offset += status.len across an allocation boundary.

"Dirty" has one meaning across formats: the node holds writes that are not yet part of its durable base. Raw reports exact dirty 512-byte sectors, a CHD diff reports a coarse 1/0. commit folds the layer into the base (raw in place, CHD by rebuilding from the merged view and reopening); rollback discards it and reverts reads to the base.

Migration

• ScsiDevice::open stays the single constructor both controllers use (the seam from scsi: targets any controller can open, with the machine's identity #265), so the WD33C93A and AIC-7880 paths need no call-site change and the AIC keeps its CHD/COW support. It now builds nodes.
• The GUI cow commit path uses CowNode; chd-extract uses ChdNode.
• chd_disk gains ChdHd::sector_size and ChdCd::read_at; cow_disk exposes the sidecar helpers the node shares.
• DiskBackend is deleted.

Preserved exactly

• ScsiIdentity (scsi: targets any controller can open, with the machine's identity #265) and the controller-provided INQUIRY identity.
• Allocation-length truncation (scsi: cut data-in replies to the allocation length #279) and the zero-length REQUEST SENSE = 4 bytes rule (scsi: a REQUEST SENSE allocation length of 0 asks for 4 bytes #283); their tests are untouched.
• Existing COW/CHD behaviour: compressed CHD diffs, COW-on always overlays, pending-sync auto-fold on a clean exit, and the overlay export/import hooks the snapshot code already uses.

Not in this PR

Physical block devices, bulk DMA, the advisory-lock node (#90) and O2-specific follow-ups. The overlay export/import methods remain on the trait only because ScsiDevice already exposes them to the snapshot code; there are no snapshot format changes here. cow_disk::CowDisk also remains as a standalone, test-covered type now that SCSI no longer uses it; deleting the old API can be a follow-up if you want it gone.

Decisions I would like comment on

• block_status is in this PR because the COW/commit contract is built on it (the issue left this open). It can be split out if you would rather land the rest first.
• I kept ScsiDevice::open rather than inlining backend construction into each controller. It preserves scsi: targets any controller can open, with the machine's identity #265's "targets any controller can open" seam and means the AIC-7880 keeps full CHD/COW support instead of a raw-only shim. Say if you would rather have node construction at the controller.
• export_overlay/import_overlay are on the trait to keep the existing snapshot hooks working without downcasting.

Tests

• cargo test --workspace: 1366 passed, 0 failed, 25 ignored.
• cargo test --workspace --features jitv2: 1848 passed, 0 failed, 31 ignored.
• src/block_node.rs: 16 unit tests — raw round-trip and bounds, COW read-through/overlay/discard/no-backing, a block_status run straddling allocated and held sectors, partial-sector read-modify-write, CHD read/write, dirty-sidecar reopen, export/import, and commit/rollback through dyn BlockNode for both raw and CHD, plus CHD pending-sync and overlay paths. The fork's advisory-lock regression test is deliberately not included because that node is out of scope, so 16 of its 17.
• scsi::tests: the three scsi: cut data-in replies to the allocation length #279/scsi: a REQUEST SENSE allocation length of 0 asks for 4 bytes #283 tests plus two new construction-path tests (direct and COW through the SCSI READ/WRITE path). chd_disk and cow_disk suites are unchanged and green.

Provenance

This is a port of the irix7/iris implementation by Jupiter Belic — commits df7f6c7, 1801c06 and d9c1953 — adapted to upstream's current ScsiIdentity/open seam. The later advisory-lock work (ab0be92) is not included.