library: five protocols speak the envelope

The folded header stops being a rule in a document and becomes the layout
on the wire. Verbs number from sixteen, leaving describe, enumerate,
subscribe and unsubscribe reserved and answered the same way by every
provider — none of them writes a line to do it. What each protocol used to
carry in a field of its own now travels in the header: a vfs node and a
display layer are the packet's target, and a reply opens with a status the
envelope stamps rather than one each protocol spelled for itself.

Display gains the most. One forty-byte request had served eleven verbs, so
attach_scanout smuggled stride through x, refresh through y and format
through colour, and every coordinate crossed as a bitcast. Per-operation
structs end all three: the fields have their own names and their own signs,
and the tile payload grows to 224 bytes because the prefix shrank. Scanout
loses a message maximum of 64 it had no business declaring — it answers
calls, and the floor for a call is 256 — and virtio-gpu stops hard-coding
that number at its harness.

Two changes are semantic rather than notational. A directory now ends at an
entry with no name, because the fixed part of a reply always travels and a
zero-length reply no longer exists to mean anything. And input joins the
service harness, the last loop in the tree that answered no ping and heard
no terminate; its subscriber table, its pruning and its fan-out are the
same code, and a shutdown now asks it to stop instead of killing it.

A new conformance case reads the registry's own listing and asks every
protocol it finds for its name, its version and its verb count, then offers
a verb nobody defines and requires -ENOSYS — the envelope's promise,
checked against providers rather than against itself. What it cannot reach
in that boot it names on the serial line instead of passing quietly.

Suite 110/110.

system/configuration/protocol.csv

@@ -148,6 +148,16 @@
 /test/system/services/input-source,          kernel,                          open,       input
 /test/system/services/input-test,            kernel,                          open,       input
 
+# The conformance probe (test/system/services/protocol-conformance-test) asks
+# every provider its boot bound for the envelope's reserved verbs. It reaches
+# ONLY the two contracts its own scenario boots a provider for, named one at a
+# time exactly like the two rows above — no subtree, no wildcard. Everything else
+# under /protocol stays absent for it, which is the point: the fixture walks the
+# namespace listing and reports what it could not open rather than being handed
+# the tree to make the test look broad.
+/test/system/services/protocol-conformance-test, kernel,                      open,       input
+/test/system/services/protocol-conformance-test, kernel,                      open,       display
+
 # The laundering-deputy probe (test/system/services/protocol-registry-test) runs
 # a grandchild whose supervisor is a fixture nobody authorized — that is the
 # point of it, and its bind must stay refused. It still has to report the verdict