M15: interrupts for PCI devices (ECAM config space + MSI)

Two parts, both blockers for real PCI drivers.

ECAM config space per function: enumeratePci now gives every pci_device its own 4 KiB
configuration window as resource 0. A claimed PCI driver mmio_maps that to reach its
command register, BARs, and — the point — its capability list (MSI/MSI-X, PCIe extended
caps), with no new syscall. Verified in the discovery test (QEMU q35's functions each
carry it).

MSI: msi_bind(device_id, endpoint) -> address (rax), data (rdx) allocates a per-device
edge-triggered vector, binds it to the endpoint, and returns the (address, data) the
driver programs into its own MSI capability. Unlike irq_bind there's no GSI, no I/O APIC
entry, no sharing, and no ack cycle — dispatch recognises an MSI vector (vector_gsi ==
none, msi_bound set), EOIs, and notifies. Owner-keyed release drops the binding on exit.
Legacy INTx (_PRT parsing + shared lines) is deliberately skipped; MSI is the real
answer.

QEMU's HPET has no MSI, so the new `msi` test proves the vector-routing path with a
self-IPI (new apic.selfIpi) standing in for the device's MSI write: bind a vector,
fire it, the bound endpoint is notified. The msi_bind syscall wraps irq.msiBind with
the claim check and lands its first real use with the first PCI driver. Suite 39/39
plus host tests.

system/devices/acpi.zig

@@ -564,6 +564,15 @@ fn enumeratePci(
                     bridge.name(), bus, device, function,
                 }) catch "pcidev";
                 const node = try device_tree.addChild(bridge, .pci_device, nm);
+                // Resource 0 is the function's own 4 KiB ECAM configuration space. A
+                // claimed PCI driver mmio_maps this to reach its command register,
+                // BARs, and — the point — its capability list (MSI/MSI-X, PCIe
+                // extended caps), without any new syscall. Physical address per the
+                // ECAM formula (same as pciConfigurationPtr).
+                const config_physical = alloc.base_address +
+                    (@as(u64, @as(u8, @intCast(bus)) - alloc.start_bus) << 20) +
+                    (@as(u64, device) << 15) + (@as(u64, function) << 12);
+                _ = node.addResource(.memory, config_physical, abi.page_size);
                 node.ids.pci_vendor = h.vendor_id;
                 node.ids.pci_device = h.device_id;
                 node.ids.pci_class = (@as(u24, h.class_code) << 16) |