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.

test/system/services/protocol-conformance-test/build.zig

@@ -0,0 +1,28 @@
+//! The protocol-conformance-test fixture as a binary package (docs/build-packages-plan.md):
+//! this file names the binary and EXACTLY the modules its source imports —
+//! build-support resolves each name from the domains this zon declares.
+
+const std = @import("std");
+const build_support = @import("build-support");
+
+pub fn build(b: *std.Build) void {
+    const exe = build_support.userBinary(b, .{
+        .name = "protocol-conformance-test",
+        .root_source_file = b.path("protocol-conformance-test.zig"),
+        .imports = &.{
+            "block-protocol",
+            "channel",
+            "display-protocol",
+            "envelope",
+            "file-system",
+            "input-protocol",
+            "ipc",
+            "logging",
+            "process",
+            "scanout-protocol",
+            "time",
+            "vfs-protocol",
+        },
+    });
+    b.installArtifact(exe);
+}