swarm: document the single-writer invariant (Rule 4) in README

lang/swarm/README.md

@@ -41,11 +41,37 @@ fn is directly threadable — the worker entry is exactly such a fn.
 | File | Framework grounding | What it does |
 |------|--------------------|--------------|
 | `worktrack.el` | Swarm §6 (correlation IDs, audit) | Durable, single-writer **JSONL journal** keyed by correlation ID; reconstructable status report; opt-in engram mirror (`SWARM_MIRROR=1`). |
-| `containment.el` | Swarm §3 (the three rules) | Scope tokens; **Rule 1** (no join), **Rule 2** (no open), **Rule 3** (no lateral edge) enforced as checks. |
+| `containment.el` | Swarm §3 + the single-writer invariant | Scope tokens w/ capabilities; **Rule 1** (no join), **Rule 2** (no open), **Rule 3** (no lateral edge), **Rule 4** (engram-write is @manager-only, by capability) enforced as checks. |
 | `ccr.el` | CCR §5 + Swarm §9.3 | Per-worker **Compiled Context Routing**: retrieve → scope → compact into a **bounded, minimal** package. The compiled-context boundary *is* the security boundary. |
 | `primitives.el` | CCR §2 (Five Primitives) | `attend / think / intend / act / learn` seam the swarm composes over. Engram-backed; explicit binding point for the API-surface reshape. |
 | `swarm.el` | Swarm §2, §4, §5 | The coordinator: fan-out/converge on native threads, bounded concurrency, four convergence strategies, integer failure threshold, full tracking. |
 
+## Invariant: only the orchestrator mutates global engram state
+
+**Only the orchestrator (@manager) writes to the engram / mutates global state.
+Workers are read-only against the full engram and may write only their own local
+geometry (their returned result + the journal). A worker is STRUCTURALLY UNABLE
+to mutate global engram state.**
+
+This is **Rule 4** — an **authority gate, not a health gate**. Scope tokens carry
+a capability set: the orchestrator's token holds `engram:write` + `dharma:emit`
+(@manager-only, the VBD rule that only the manager mutates global state); a
+worker's token holds **only** `engram:read`. Every engram mutation
+(`op_write`/`op_relate`/`op_supersede` → `POST /api/nodes`, `/api/edges`,
+`DELETE`) flows through `swarm_engram_write`, which checks the caller's capability
+via the **same scope-token mechanism as the live Rule-2 denial** and rejects any
+worker **before any HTTP is issued**. Capability is fixed at mint time and cannot
+be acquired at runtime — so the guarantee holds regardless of engram health
+(distinct from the `SWARM_WRITE_HEALTHY` *health* gate).
+
+The **curated merge is the only write path**: workers return geometry; the
+orchestrator, and only the orchestrator, commits the approved/verified geometry
+back (`commit=1`). Workers keep full-engram **read** access (`op_think`/`op_read`).
+
+Proven in `harness_real_cognition.el` (§G): a worker `swarm_engram_write` is
+DENIED by capability with no node created and the violation journalled; the
+orchestrator passes the gate as the sole authorized writer.
+
 ## Containment → distribution
 
 The three containment rules make workers **location-independent** (Swarm §9): a