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/routing-policy.json

@@ -0,0 +1,25 @@
+{
+  "routing_policy": {
+    "default_revision": "R-000220",
+    "generation": 118,
+    "id": "rp-118",
+    "interface": "customer-finance-api",
+    "issued_at": "2026-09-05T08:00:00Z",
+    "issued_by": {
+      "id": "fluid-experiment-controller",
+      "type": "system"
+    },
+    "rules": [
+      {
+        "allocation": {
+          "R-000220": 0.8,
+          "R-000221": 0.2
+        },
+        "cohort": "coding-agents",
+        "experiment": "E-000093",
+        "sticky_by": "consumer_id"
+      }
+    ],
+    "schema_version": "0.1"
+  }
+}