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/event.yaml

@@ -0,0 +1,24 @@
+fluid_event:
+  schema_version: "0.1"
+
+  id: "EV-990281"
+  occurred_at: "2026-09-09T11:31:02Z"
+
+  entity_type: "revision"
+  entity_id: "R-000221"
+
+  event_type: "PROMOTION_RECOMMENDED"
+
+  actor:
+    type: "daimon"
+    id: "fluid-daimon/customer-finance-api"
+
+  inputs:
+    - "H-000184"
+    - "E-000093"
+
+  reason: >
+    Candidate met primary success target and violated no guardrails.
+
+  evidence_refs:
+    - "metrics:E-000093/window-4"