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/routing-policy.yaml

@@ -0,0 +1,20 @@
+routing_policy:
+  schema_version: "0.1"
+
+  id: "rp-118"
+  interface: "customer-finance-api"
+  generation: 118
+  issued_at: "2026-09-05T08:00:00Z"
+  issued_by:
+    type: "system"
+    id: "fluid-experiment-controller"
+
+  default_revision: "R-000220"
+
+  rules:
+    - cohort: "coding-agents"
+      experiment: "E-000093"
+      allocation:
+        R-000220: 0.80
+        R-000221: 0.20
+      sticky_by: "consumer_id"