library: the envelope — one packet shape for every protocol

A protocol is now defined through envelope.Define rather than beside it.
Every packet begins with the same 16-byte prefix: a Header of {operation,
target} for a request or event, a Status for a reply. The header is folded
into each protocol's own layout, never stacked on top of it — Define walks
a spec's types and refuses one that carries a Header or Status field, so
the rule cannot be missed by reading. Operations number from 16, leaving
describe, enumerate, subscribe and unsubscribe reserved and answerable the
same way everywhere; describe the envelope answers itself, and an unknown
verb is -ENOSYS without a provider writing a line.

The size floor becomes something protocols can compile against: 256 bytes
for a call, 64 for a push, matching the kernel's own limits, checked at the
Define call with the prefix counted in. A protocol that outgrows either
fails the build with the reason and the remedy — shrink, or move the bulk
to shared memory, because packets never fragment. Events carry the header
too and number in their own space, so appending an operation can never
renumber a shipped event.

Beside it, the client half of the model: a Channel is what opening a
/protocol name yields, wrapping the endpoint capability so a program holds
a conversation rather than a handle. Nothing speaks through either yet —
the vfs protocol gains NodeKind.protocol and the convention that an open
reply may carry a capability, and that is the whole runtime change.

Suite 107/107, unchanged: P1 converts nothing.

library/protocol/build.zig

@@ -3,6 +3,10 @@
 //! every conversation depend on the contract by name; neither reaches into the
 //! other's files. Pure flat wire types: no protocol module imports anything.
 //!
+//! One module here is not a protocol but the shape the others are written in:
+//!
+//!   envelope                 : the packet prefix + comptime Define (docs/os-development/protocol-namespace.md)
+//!
 //!   vfs-protocol             : the VFS server <-> the file layer (unistd/stdio)
 //!   input-protocol           : the input fan-out service <-> sources + subscribers
 //!   block-protocol           : a filesystem <-> a block driver (usb-storage)
@@ -16,6 +20,9 @@ const std = @import("std");
 
 pub fn build(b: *std.Build) void {
     for ([_]struct { name: []const u8, root: []const u8 }{
+        // Not a protocol, hence not `-protocol`: the envelope is what a
+        // protocol is defined *through*.
+        .{ .name = "envelope", .root = "envelope/envelope.zig" },
         .{ .name = "vfs-protocol", .root = "vfs/vfs-protocol.zig" },
         .{ .name = "input-protocol", .root = "input/input-protocol.zig" },
         .{ .name = "block-protocol", .root = "block/block-protocol.zig" },
@@ -32,6 +39,7 @@ pub fn build(b: *std.Build) void {
     // its aggregate test step.
     const test_step = b.step("test", "Run the protocol unit tests");
     for ([_][]const u8{
+        "envelope/envelope.zig", // framing round trips, verb numbering, dispatch, the floors
         "vfs/vfs-protocol.zig", // NodeKind / DirectoryEntry sizes + op values
         "display/display-protocol.zig", // pack(): native pixel encoding per format
     }) |root| {