docs: document the supervision hierarchy and driver discovery

drivers.md gains a "How a driver gets started" section — the doc explained what a
running driver does but never who starts it. It lays out the three-level supervision
hierarchy (kernel spawns init; init spawns the services; the device-manager discovers,
matches, and spawns the drivers) and names the two user-space policies that "configure"
drivers today: init's service list and the device-manager's match table.

driver-model.md's "what exists today" adds system_spawn and the supervision model, and
the drivers.md restart bullet is refreshed: a spawning supervisor now exists, a
restarting one still doesn't.

docs/drivers.md

@@ -20,6 +20,49 @@ them for itself:
 A driver is, in one sentence, *a process that sleeps until its device has something to
 say.*
 
+## How a driver gets started: discover, match, spawn
+
+Nothing in the kernel decides that the HPET needs the `hpet` driver — that is policy,
+and policy lives in user space. Boot brings user space up as a three-level supervision
+hierarchy, each level owning one job:
+
+```
+kernel  ──spawns──►  init (PID 1)  ──spawns──►  device-manager  ──spawns──►  hpet
+  |                     |                          |
+  spawns only init,     the service supervisor:    the driver supervisor: enumerates
+  publishes the         starts the system          /system/devices, matches each device
+  initial-ramdisk       services (vfs, the         to a driver, and system_spawn's it
+  so user space can     device-manager). Its
+  system_spawn from it  list is init policy.
+```
+
+The kernel launches exactly one process — `init` — and hands it nothing but the raw
+ability to start more (`system_spawn(name)`, which loads a binary bundled in the
+initial-ramdisk as a fresh ring-3 process). Everything else is a user-space decision:
+
+- **init** ([system/services/init](system/services/init/init.zig)) is the **service
+  supervisor**. It spawns the system services danos brings up at boot — today `vfs` and
+  the `device-manager` — from a small list. Drivers are deliberately *not* its job.
+- **device-manager** ([system/services/device-manager](system/services/device-manager/device-manager.zig))
+  is the **driver supervisor**. It does the three steps a monolithic kernel would do in
+  its probe path, entirely from ring 3:
+  1. **Discover** — `device_enumerate` snapshots the device table the kernel built from
+     ACPI/PCI ([discovery](discovery.md)).
+  2. **Match** — for each device it looks up a driver by `DeviceClass`. The match policy
+     is a table (`driverFor`): today a static `timer → hpet` map; a fuller system reads
+     what each driver *binds* (a manifest under `/system/drivers`, or the driver
+     describing its own match).
+  3. **Spawn** — `system_spawn(driver_name)` starts the matched driver, which then claims
+     its device and runs the event loop below.
+
+So "how is a driver discovered and configured" has two halves: **discovery** is the
+kernel's device table, read by anyone; **configuration** is two user-space policies —
+init's service list and the device-manager's match table. Both are hardcoded in their
+respective programs today; the natural next step is to move them into `/etc` (see the
+milestone notes in [driver-model.md](driver-model.md)). `system_spawn` is currently
+ungated — any process may spawn any bundled binary — because there is no spawn
+capability yet.
+
 ## The capability: claim before touch
 
 The driver syscall numbers (`system/abi.zig`) with the device types they carry
@@ -312,9 +355,12 @@ controller drivers), and the IOMMU — have proposed signatures in
 - **Unregistering children.** `device_register` only appends. A USB device that is
   unplugged cannot be removed, and a bus driver in a loop can exhaust the 64-entry
   table.
-- **Restart.** A driver that dies should release its claim, have its device quiesced,
-  and be respawned by a supervisor. Some pieces (`releaseIrqs`, `device_grant`
-  teardown, the claim table) exist; the policy doesn't.
+- **Restart.** A supervisor that *spawns* drivers now exists — the device-manager starts
+  them with `system_spawn` — but a supervisor that *restarts* them does not. A driver that
+  dies should release its claim, have its device quiesced, and be respawned; today nothing
+  notices the death. Some pieces (`releaseIrqs`, `device_grant` teardown, the claim table)
+  exist, and `dev_release` (below) is the missing mechanism; the restart policy is the
+  resilience track ([resilience.md](resilience.md)).
 - **Interrupt priority / threaded IRQ latency.** `notifyFromIsr` enqueues the woken
   driver but doesn't preempt (`wakeLocked` deliberately leaves that to the caller), so
   a woken driver waits for the next scheduling point.