library: the harness keeps the subscribers, and an id belongs to whoever opened it

Three services had each written the same thing and got it three different
ways: input polled the process list to notice a dead subscriber, and only
when someone else subscribed; the power service never noticed at all; the
device manager noticed drivers but not subscribers. The harness owns the
table now, driven by the events a protocol declares — it registers on the
reserved verb, frames each event once, posts to everyone interested without
waiting on any of them, and reclaims a slot when the kernel says its owner
died. Interest masks moved to the envelope, so a subscriber that wants only
mice asks the same way everywhere.

Two consequences the plan had not foreseen. The device manager now hears a
supervised child's death twice, once as its supervisor and once as a
subscriber, so restart backoff counted every crash twice and gave up after
half as many; it retires the id before counting. And the kernel's published
exit table had eight slots for what is now six subscriptions in a plain
boot, so it holds sixteen.

The other half is a hole the design named early and left standing: a
backend handed out a small integer and then honoured it from anyone. A
process that guessed a file's node id read another client's file; a display
layer had no owner at all, so any client could reconfigure or destroy any
layer; a USB device token was never checked against the client that opened
it. Each is now bound to the task that opened it, and a wrong owner gets
exactly what an unknown id gets — the refusal must not become the oracle
the identical answers elsewhere were designed to remove. Closing a file
changed with it: it used to succeed unconditionally, which would have told
a caller which ids existed.

Suite 111/111, with a new case in which one process holds a file and a
layer, hands both ids to a second process, and finds them untouched after
that process has tried everything with them.

docs/device-driver-development/device-manager.md

@@ -81,6 +81,16 @@ one world.
 | app → manager | `subscribe` (reserved verb 2) | receive published add/remove events; the subscriber's endpoint rides as the call's capability |
 | manager → app | `child_added` / `child_removed` events | the same two structs, pushed rather than called |
 
+The watcher table behind those last two rows is the **service harness's**
+(`service.Subscribers`, shared with input and power), not the manager's: it
+answers `subscribe`/`unsubscribe`, frames each event once for the fan-out, and
+sweeps a watcher on its exit notification — where the manager previously had no
+sweep for watchers at all. Its own supervised-driver exits are a different thing
+and unchanged, except that a driver's death now arrives twice (the manager is
+both its supervisor and a subscriber to published exits), so the manager retires
+a dead driver's process id as it handles the first and the second finds nothing
+to act on.
+
 Two of what used to be the manager's own operations are the envelope's **reserved**
 verbs, which mean the same thing at every provider in the system, so this protocol
 numbers only three of its own (`hello` = 16, `child_added` = 17,