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/experiment.json

@@ -0,0 +1,44 @@
+{
+  "fluid_experiment": {
+    "allocation": {
+      "candidate": 0.28,
+      "control": 0.72
+    },
+    "candidate_revisions": [
+      "R-000221"
+    ],
+    "cohorts": [
+      "partner-integrations",
+      "coding-agents"
+    ],
+    "control_revision": "R-000220",
+    "hypothesis_refs": [
+      "H-000184"
+    ],
+    "id": "E-000093",
+    "interface_id": "customer-finance-api",
+    "metrics": {
+      "guardrails": [
+        "p95_latency_ms",
+        "error_rate",
+        "authorization_scope_change"
+      ],
+      "primary": [
+        "requests_per_completed_task"
+      ]
+    },
+    "planned_end_at": "2026-10-05T08:00:00Z",
+    "result": {
+      "evidence_refs": [],
+      "preferred_revision": null,
+      "state": "RUNNING"
+    },
+    "schema_version": "0.1",
+    "start_at": "2026-09-05T08:00:00Z",
+    "stop_conditions": [
+      "hard_guardrail_violation",
+      "security_failure",
+      "manual_stop"
+    ]
+  }
+}