Generate contract types, pin fixtures to spec, add build and ADRs

Completes FLUID-WP-0002. The record types in internal/contract are
generated from schemas/ by a dependency-free generator; fixtures
transcribed from the spec's worked examples validate against those
schemas and round-trip through the generated types with
DisallowUnknownFields, so a spec change that misses the schemas fails
CI rather than drifting silently.

Adds identifier prefix helpers, Makefile, GitHub Actions, and five ADRs
recording the Go choice, out-of-process attachment, the wire contract as
boundary, the evidence store, and revision identity.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014KmVxhJ35tCo7rE7UnLwWu

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 1116572@bnt-lap001
Assistant-Session: 8ba9bb93-a72a-4883-b189-2499cce5c400

conformance/fixtures/json/backend-requirement.json

@@ -0,0 +1,38 @@
+{
+  "fluid_backend_requirement": {
+    "backend_service": "payments-service",
+    "capability": {
+      "description": "Retrieve payment state as of a specified timestamp.\n",
+      "title": "Historical payment-state query"
+    },
+    "disposition": {
+      "reason": null,
+      "state": "PLANNED",
+      "target_ref": "payments-service/roadmap#412"
+    },
+    "expected_usage": {
+      "requests_per_day": 120000
+    },
+    "id": "BR-0041",
+    "originating_hypothesis": "H-0212",
+    "originating_interface": "customer-context-api",
+    "originating_revision": "R-0081",
+    "quality": {
+      "availability": 0.999,
+      "p95_latency_ms": 250
+    },
+    "schema_version": "0.1",
+    "security": {
+      "authorization_scope": "payments.read",
+      "tenant_isolation": "required"
+    },
+    "semantics": {
+      "consistency": "snapshot",
+      "required_fields": [
+        "payment_id",
+        "state",
+        "effective_at"
+      ]
+    }
+  }
+}