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/backend-requirement.yaml

@@ -0,0 +1,37 @@
+fluid_backend_requirement:
+  schema_version: "0.1"
+
+  id: "BR-0041"
+  originating_interface: "customer-context-api"
+  originating_revision: "R-0081"
+  originating_hypothesis: "H-0212"
+
+  backend_service: "payments-service"
+
+  capability:
+    title: "Historical payment-state query"
+    description: >
+      Retrieve payment state as of a specified timestamp.
+
+  semantics:
+    consistency: "snapshot"
+    required_fields:
+      - "payment_id"
+      - "state"
+      - "effective_at"
+
+  quality:
+    p95_latency_ms: 250
+    availability: 0.999
+
+  security:
+    authorization_scope: "payments.read"
+    tenant_isolation: "required"
+
+  expected_usage:
+    requests_per_day: 120000
+
+  disposition:
+    state: "PLANNED"
+    reason: null
+    target_ref: "payments-service/roadmap#412"