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.

docs/security-track-plan.md

@@ -35,7 +35,7 @@ the logging/USB-lifecycle track).
 - [x] **PM** — path-migration flag-day (`/etc`→`/system/configuration`, `/var/log`→`/system/logs`, `/mnt/usb`→`/volumes/usb`; vfs carve-out for the two writable `/system` subtrees, FAT's `/var` mount split in two; suite 106/106)
 - [x] **H1** — the `user-memory` module; nine stragglers converted; leaf U/S+W checks (plus physmap-coverage confirmation, so an `mmio_map`'d buffer cannot fault ring 0 — this also closes the same hazard on the IPC path; `fs_resolve`'s out-capacity bound made overflow-safe; suite 107/107)
 - [x] **merge** group 1 → main, push (f3bc23c, 2026-07-31)
-- [ ] **P1** — envelope module + `Define`; vfs `NodeKind.protocol` + open-reply-capability; client `Channel`
+- [x] **P1** — envelope module + `Define`; vfs `NodeKind.protocol` + open-reply-capability; client `Channel` (mechanics only, nothing converted; suite unchanged at 107)
 - [ ] **P2** — registry in init; `/protocol` reserved; ServiceId flag-day (11 binds, 17 lookups)
 - [ ] **P3** — open grants: `protocol.csv` enforcement, denial test
 - [ ] **merge** group 2 → main, push