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/event.json

@@ -0,0 +1,22 @@
+{
+  "fluid_event": {
+    "actor": {
+      "id": "fluid-daimon/customer-finance-api",
+      "type": "daimon"
+    },
+    "entity_id": "R-000221",
+    "entity_type": "revision",
+    "event_type": "PROMOTION_RECOMMENDED",
+    "evidence_refs": [
+      "metrics:E-000093/window-4"
+    ],
+    "id": "EV-990281",
+    "inputs": [
+      "H-000184",
+      "E-000093"
+    ],
+    "occurred_at": "2026-09-09T11:31:02Z",
+    "reason": "Candidate met primary success target and violated no guardrails.\n",
+    "schema_version": "0.1"
+  }
+}