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/experiment.yaml

@@ -0,0 +1,41 @@
+fluid_experiment:
+  schema_version: "0.1"
+
+  id: "E-000093"
+  interface_id: "customer-finance-api"
+
+  hypothesis_refs:
+    - "H-000184"
+
+  control_revision: "R-000220"
+  candidate_revisions:
+    - "R-000221"
+
+  cohorts:
+    - "partner-integrations"
+    - "coding-agents"
+
+  allocation:
+    control: 0.72
+    candidate: 0.28
+
+  start_at: "2026-09-05T08:00:00Z"
+  planned_end_at: "2026-10-05T08:00:00Z"
+
+  metrics:
+    primary:
+      - "requests_per_completed_task"
+    guardrails:
+      - "p95_latency_ms"
+      - "error_rate"
+      - "authorization_scope_change"
+
+  stop_conditions:
+    - "hard_guardrail_violation"
+    - "security_failure"
+    - "manual_stop"
+
+  result:
+    state: "RUNNING"
+    preferred_revision: null
+    evidence_refs: []