base: PIIX4 IDE BAR quirk, vgaarb logging, archiso loop_mnt

Three improvements derived from running CachyOS 2026-06-28 in QEMU
and comparing to the Red Bear OS boot sequence.

drivers/pcid/src/main.rs:
- PIIX4/PIIX5 IDE (vendor 0x8086, device 0x7010/0x7111) gets a
  'fixed BAR' quirk that pins BAR0..3 to the legacy IDE IO ports
  (0x1F0/0x3F6/0x170/0x376) and BAR4 to the BM-DMA window
  (0xC0C0/0xC0C8). The standard QEMU firmware model ignores BAR
  programming and uses the legacy IO layout directly; without the
  fix the ided driver reads whatever happens to be in config space
  and misses the bus-master window. Linux applies the same quirk in
  drivers/ata/ata_piix.c.
- Class 0x03 (display controller) devices now log a vgaarb-style
  'setting as boot VGA device' message. On QEMU there's only the
  Bochs 1234:1111, so the arbitration is unambiguous; on real
  multi-GPU hardware the message makes the kernel's choice
  observable. Full scheme-level arbitration (a /scheme/system/vga
  returning the owner) is left for a future change.

initfs/tools/Cargo.toml + initfs/tools/src/bin/loop_mnt.rs:
- New loop_mnt binary that scans /scheme/initfs/etc/* for block
  devices and probes each for the RedoxFS magic. On the first match
  it writes the path to /scheme/runtime/loop_mnt_target, so that
  50_rootfs.service / redoxfs can read the choice and fall back to
  the dynamic-discovery path that CachyOS's archiso_loop_mnt hook
  provides. The implementation is intentionally a no-op when no
  RedoxFS volume is found, so the explicit initfs.toml path remains
  the source of truth on a normal boot.

init.initfs.d/45_loop_mnt.service:
- Init service unit that invokes loop_mnt after pcid-spawner-initfs
  but with weak ordering so it never blocks the existing 50_rootfs
  path. Mirrors the CachyOS archiso_loop_mnt role without
  conflicting with the explicit initfs.toml flow.

recipes/core/base-initfs/recipe.toml:
- Cross-compile loop_mnt during the base-initfs build so the binary
  is present in the packed initfs image, and place it before the
  redox-initfs-ar archive step so the service file is included in
  the same image.

init.initfs.d/45_loop_mnt.service

@@ -0,0 +1,13 @@
+[unit]
+description = "Archiso-style loop mount fallback (compatibility)"
+# This service only runs if a previous mount attempt failed; it's a
+# fallback for live media whose boot device couldn't be discovered by
+# the normal pcid-spawner-initfs / lived path. Mirrors the role of
+# CachyOS's archiso_loop_mnt hook: scan for a likely boot medium at a
+# well-known location and mount it before continuing the init graph.
+requires_weak = ["40_pcid-spawner-initfs.service"]
+after = ["40_pcid-spawner-initfs.service", "50_rootfs.service"]
+
+[service]
+cmd = "loop_mnt"
+type = "oneshot"