parport-pci: NetMos MCS9900 PCI parallel port with IEEE 1284 reverse

[SolAstrius]

Adds a NetMos / MosChip MCS9900 PCI parallel port — a single SPP port plus the IEEE 1284 nibble-mode reverse channel — and wires up CLI flags to drive it. Builds on the I/O-port BAR + per-function subsystem-ID support merged in #225.

Device

PCI identity matches the MCS9900 datasheet (ASIX Rev 2.00, §6.2, the 2S+1P parallel function): vendor 0x9710, device 0x9900, class 0701h, subsystem 0xA000:0x2000. BAR 0 is an I/O-port BAR (PCI Local Bus Spec 3.0 §6.2.5.1) holding the 8-byte SPP register window (Data / Status / Control + EPP/ECP stubs). The subsystem IDs (PCI §6.2.4) are what a host driver strict-matches on to bind.

• Forward path — standard Centronics (IEEE 1284 §7.3): the guest writes Data, then pulses the strobe; we forward the byte to a backend callback on the strobe edge.
• Reverse path — IEEE 1284 §7.4 negotiation + §6.3 nibble mode: a small state machine tracks the negotiation events, presents peripheral bytes a nibble at a time on the status lines, and raises an edge INTx on nAck when IRQ is enabled. A lock-protected ring buffers host→guest bytes; nFault signals end-of-data when it drains.

CLI

A single -parport <spec> flag (structured value, like -portfwd/-res):

• -parport null — attach with no backend.
• -parport <path> — bare path shorthand; guest output is sinked to that file.
• -parport out=<path>,in=<path> — out= sinks guest (forward/Centronics) output to a file/pipe; in= sources the IEEE 1284 reverse channel from a file/fifo (a reader thread feeds bytes into the ring). Either field may be omitted.

Notes

• We expose prog_if 0x00 (SPP); the silicon reports 0x03 (IEEE 1284 controller). SPP is sufficient for a host driver to bind and use the nibble reverse channel.
• parport_pci_remove clears the instance from the lookup map before freeing, so a reused pci_dev_t address can't resolve to a freed device.

Test plan

Verified on an Alpine RISC-V guest (kernel with parallel-port + ppdev support):

• enumerates with the expected IDs and I/O BAR; parport / lp / ppdev nodes appear
• echo to the printer node lands in the host sink file
• nibble-mode reverse reads (-parport_in → guest cat) deliver bytes intact