M10: IO passthrough (MMIO grants) + first real driver (hpetd)

A user-space process can now touch real hardware directly, capability-gated by
the device tree — the microkernel driver model.

- src/kernel/devsvc.zig: flattens the discovered device tree into an
  id-indexed snapshot + a claim table at boot (devsvc.init from main.zig).
- Syscalls 11-13: dev_enumerate (snapshot the table), dev_claim (take
  exclusive ownership), mmio_map (map a claimed device's MMIO window into the
  caller's AS and return the register base). The claim is the capability:
  mmio_map refuses any device the caller doesn't own.
- paging.mapUserDeviceInto: maps device MMIO strong-uncacheable (PCD|PWT) and
  marks each leaf with a device_grant PTE bit; freeSubtree skips pmm.free on
  those leaves, so tearing down a driver never returns MMIO frames to the RAM
  pool (the teardown hazard). MMIO grants live in a distinct arena, PML4[226]
  (Task.dev_map_next), so device pages widen no kernel mapping.
- lib/dev.zig: user enumerate/claim/mmioMap wrappers; shared DeviceDesc/ResDesc
  in danos (root.zig). sbin/hpetd.zig: finds the HPET, claims it, maps its
  registers, enables the counter (an MMIO write) and reads it (0xF0) — proving
  read+write passthrough to real hardware.
- Tests: `hpet` (driver reads the counter advancing from ring 3) and `iopass`
  (device-granted frame survives address-space teardown). Suite 33/33.

irq_bind/irq_ack (IRQ-as-message) are stubbed (-1) pending; notifyFromIsr (M7)
is the hook they'll use.

test/qemu_test.py

@@ -188,6 +188,16 @@ CASES = [
     {"name": "vfs",
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # IO passthrough: a user-space HPET driver maps device MMIO into its own
+    # address space and reads the counter advancing.
+    {"name": "hpet",
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # The MMIO-grant teardown fix: a device-granted frame must not be reclaimed
+    # as RAM when its address space is destroyed.
+    {"name": "iopass",
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # The ACPI power path succeeds by QEMU *exiting* (S5 off / reset), so match the
     # pre-transition marker; the FAIL line only appears if the transition didn't take.
     {"name": "poweroff",