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.

system/services/display/build.zig

@@ -10,9 +10,9 @@ pub fn build(b: *std.Build) void {
         .name = "display",
         .root_source_file = b.path("display.zig"),
         .imports = &.{
-            "channel", "display-client", "display-protocol", "driver",  "envelope", "input-client",
-            "ipc",     "logging",        "memory",           "scanout-protocol", "service",
-            "thread",  "time",
+            "channel", "display-client", "display-protocol", "driver",           "envelope", "input-client",
+            "ipc",     "logging",        "memory",           "process",          "scanout-protocol",
+            "service", "thread",         "time",
         },
         .threaded = true, // real atomics/TLS (docs/threading.md)
     });