pci: full driver-side library (MSI/MSI-X, power, FLR, extended caps); xhci goes interrupt-driven

library/device/pci is now the complete generic floor a leaf PCI driver
needs, instead of just what virtio-gpu used:

- pci-class: capability IDs, MSI/MSI-X/power-management/PCI-Express
  register layouts, extended-capability header decode (host-tested),
  per-bit command constants, remaining header offsets.
- pci.Function: header accessors, disableBusMaster + interrupt-disable
  helpers, findCapability, programMsi/disableMsi, MsiX vector-table
  struct, ensurePowerStateD0, functionLevelReset (BAR save/restore),
  extended-capability iterator.

Proven by the new pci-caps QEMU case: a pci-cap-test fixture claims an
extra e1000e (PM+MSI+PCIe+MSI-X, no danos driver) and readback-verifies
every surface, including the first driver-side use of msi_bind.

usb-xhci-bus converts from 8 ms event-ring polling to message-signalled
interrupts: plain MSI where offered (real Intel xHC), MSI-X entry 0
otherwise (qemu-xhci has no MSI capability), byte-identical polling as
fallback. The timer survives as a 250 ms port-reconcile/lost-edge tick —
real-hardware USB2 hub debounce still needs it. MSI setup runs BEFORE
controller bring-up: QEMU's xhci only registers the MSI-X vector as used
when IMAN.IE is written while MSI-X is already enabled; interrupts are
silently dropped otherwise (real hardware does not care about the order).

101/101 QEMU cases green; real-hardware smoke passed (mouse works,
boot 2026-07-23T174805Z, plain-MSI branch, vector 33).

test/qemu_test.py

@@ -687,6 +687,18 @@ CASES = [
     # duplicates); this ordered regex asserts the drill itself over the whole serial
     # log — the backreference requires the respawn to re-scan the same count, and the
     # full-capture match is immune to the transient-line races an in-kernel poll hits.
+    # The driver-side PCI library (library/device/pci) against a real function: an extra
+    # e1000e NIC — PM + MSI + PCIe + MSI-X capabilities, claimed by no danos driver — is
+    # claimed by the pci-cap-test fixture, which exercises the capability walks, MSI
+    # programming (the first driver-side msi_bind use), the MSI-X table, power state,
+    # and FLR, printing a marker per check.
+    {"name": "pci-caps",
+     "smp":    4,
+     "timeout": 120,
+     "qemu_extra": ["-device", "e1000e"],
+     "expect": r"(?s)(?=.*DANOS-TEST-RESULT: PASS)(?=.*pci-cap-test: all checks passed)",
+     "fail":   r"pci-cap-test: FAIL|DANOS-TEST-RESULT: FAIL"},
+
     {"name": "pci-scan",
      "smp":    4,
      "timeout": 60,