ichor(D2): start the Pony wiring — broker + D1 barrier + envelope

Name the medium Ichor and scaffold it in Pony at src/ichor:
- Envelope {source,dest,provenance,payload} mirroring Ada Organ_Message
- Broker actor: register + perfusion route; Brain-bound traffic crosses D1
- Barrier (D1) admit: Pony provenance-law mirror + Ada FFI seam sketched
- StubOrgan + main smoke wiring (deliver internal, reject external)
- ichor_ada_shim.c: C/Fortran binding seam to Ada Trust_Guard (stub)
Not compiled in-env (no ponyc). Updates D2 spec to SCAFFOLD/Pony.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015hmgREHNsxYCuim33yUF2c

src/ichor/envelope.pony

@@ -0,0 +1,64 @@
+"""
+Ichor — the perfusion medium ("the blood"). D2.
+
+The one envelope every organ emits and consumes. Mirrors the Ada D1
+`Organ_Message {Source, Destination, Provenance, Payload}` so the Pony broker
+and the Ada border (Trust_Boundary) speak the same shape across the seam.
+
+Envelope is `class val`: immutable and sendable between actors.
+"""
+
+type OrganId is
+  ( DriveBox | EnergyTorus | Soul | Metacog | Brain
+  | AdaBorder | Storage | MiniRag | UnknownOrgan )
+
+primitive DriveBox
+  fun string(): String => "drive_box"
+primitive EnergyTorus
+  fun string(): String => "etr"
+primitive Soul
+  fun string(): String => "soul"
+primitive Metacog
+  fun string(): String => "metacog"
+primitive Brain
+  fun string(): String => "brain"
+primitive AdaBorder
+  fun string(): String => "ada_border"
+primitive Storage
+  fun string(): String => "storage"
+primitive MiniRag
+  fun string(): String => "mini_rag"
+primitive UnknownOrgan
+  fun string(): String => "unknown"
+
+type Provenance is ( SystemInternal | UserInput | OrganSecretion | External )
+
+primitive SystemInternal
+  fun string(): String => "system_internal"
+primitive UserInput
+  fun string(): String => "user_input"
+primitive OrganSecretion
+  fun string(): String => "organ_secretion"
+primitive External
+  fun string(): String => "external"
+
+class val Envelope
+  let source: OrganId
+  let dest: OrganId
+  let provenance: Provenance
+  let payload: String
+
+  new val create(
+    source': OrganId,
+    dest': OrganId,
+    provenance': Provenance,
+    payload': String)
+  =>
+    source = source'
+    dest = dest'
+    provenance = provenance'
+    payload = payload'
+
+  fun string(): String =>
+    source.string() + " -> " + dest.string()
+      + " [" + provenance.string() + "] " + payload