Conformance fixtures: what two implementations may not disagree about

Tasks 3.1 and 3.3 of novox/hq ADR 0116. One fixture directory, read by every implementation's own runner rather than copied into each — a fixture copied twice is two fixtures, and two fixtures drift. The README settles what ADR 0074's byte-for-byte can and cannot mean. The envelope is exact: the subject, the required headers, each name and format, because those are what two implementations get wrong invisibly. The body is not: it is the module's payload, and Go sorts a map's keys where JavaScript keeps insertion order, so demanding identical bytes would commit every implementation to a canonical JSON encoder to buy a property the mesh never uses. Said plainly, because read strictly it would have sent somebody writing one.
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+# Conformance fixtures
+
+What two implementations of the mesh's module protocol may not disagree about
+([novox/hq ADR 0074](https://git.novox.be), design 19). A fixture is data, not prose: every
+implementation produces it and consumes it, and an implementation that passes the fixtures for a
+capability **claims** that capability.
+
+## What "byte-for-byte" means, and what it cannot
+
+ADR 0074 says the fixtures must match byte for byte. Taken literally that is more than the mesh
+needs and more than two languages can give:
+
+- **The envelope is exact.** The subject, the set of required headers, each header's name, and
+  the format of each value. This is what an implementation can get wrong in a way that makes two
+  meshes silently ignore each other, so this is what is pinned.
+- **The body is not.** It is the module's own payload, opaque to the mesh, and its serialisation
+  is the language's. Go's `encoding/json` sorts a map's keys; JavaScript's `JSON.stringify` keeps
+  insertion order. Demanding identical bytes there would mean committing every implementation to
+  a canonical JSON encoder — a large promise, bought for a property the mesh never uses. The
+  fixture pins that the body **round-trips**: what goes in comes out equal.
+
+So: exact where disagreement is invisible, semantic where it is not. Stated here because
+"byte-for-byte" read strictly would have sent somebody implementing a canonical encoder.
+
+## Running them
+
+The TypeScript implementation: `node conformance/run.mjs` (against a NATS server; see the file).
+The Go implementations read this directory by sibling path, the way the lab finds its siblings.
+
+## Layout
+
+    events/            one emitted event, and what it must look like on the wire
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+{
+  "capability": "events",
+  "name": "a module emits an event",
+  "why": "The envelope is what two implementations can disagree about without either failing: a missing header, a header spelled differently, or a body nested where metadata belongs. None of those stop a mesh running; they stop it reacting.",
+  "given": {
+    "module": "shop",
+    "node": "one",
+    "key": "order.placed",
+    "body": { "id": "a1", "total": 12 },
+    "headers": {
+      "x-event-id": "0123456789abcdef0123456789abcdef",
+      "x-source": "shop",
+      "x-node": "one",
+      "x-time": "2026-09-26T12:00:00Z",
+      "content-type": "application/json"
+    }
+  },
+  "wire": {
+    "subject": "mesh.mod.shop.event.order.placed",
+    "requiredHeaders": ["x-event-id", "x-source", "x-node", "x-time", "content-type"],
+    "optionalHeaders": ["x-causation-id", "x-schema"],
+    "headerFormats": {
+      "x-time": "RFC3339",
+      "content-type": "application/json",
+      "x-event-id": "^[0-9a-f]{32}$"
+    },
+    "payloadIs": "the body alone, not the envelope",
+    "keyRecoveredFrom": "the subject, after the .event. token"
+  },
+  "refuses": [
+    {
+      "what": "an envelope nested in the payload",
+      "why": "an implementation that publishes the whole envelope as the body passes all of its own tests and is unreadable to every other"
+    },
+    {
+      "what": "a missing x-event-id",
+      "why": "delivery is at-least-once and only the emitter can say which of two messages is a redelivery"
+    },
+    {
+      "what": "an x-source that differs from the subject's module",
+      "why": "the bus enforces the namespace, so a disagreement means the envelope is lying about its origin"
+    }
+  ]
+}