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.

library/kernel/build.zig

@@ -87,11 +87,14 @@ pub fn build(b: *std.Build) void {
     });
     // The harness binds the service's contract name at startup, which is a
     // conversation with the registry — hence channel (and time, for the patience
-    // a provider that beat init to the mount needs).
+    // a provider that beat init to the mount needs). It also owns the subscriber
+    // table and the fan-out, which are expressed in the envelope's vocabulary
+    // (the reserved subscribe verb, the push floor) — hence envelope.
     _ = b.addModule("service", .{
         .root_source_file = b.path("service.zig"),
         .imports = &.{
             .{ .name = "channel", .module = channel },
+            .{ .name = "envelope", .module = protocol.module("envelope") },
             .{ .name = "ipc", .module = ipc },
             .{ .name = "process", .module = process },
         },