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/telemetry-envelope.yaml

@@ -0,0 +1,32 @@
+fluid_telemetry:
+  schema_version: "0.1"
+
+  id: "tl-01J8Z3XK7QW2"
+  occurred_at: "2026-09-05T08:14:22Z"
+  interface_id: "customer-finance-api"
+  kind: "request"
+
+  correlation_id: "c-7f21ab90"
+  consumer_ref: "psu-4d1e9c"
+  cohort: "coding-agents"
+  revision: "R-000221"
+  experiment: "E-000093"
+
+  resolution:
+    reason: "experiment_assignment"
+    policy_generation: 118
+
+  request:
+    operation: "getLatestInvoice"
+    route: "/customers/{id}/invoices/latest"
+    method: "GET"
+    status: 200
+    latency_ms: 118.4
+    request_bytes: 0
+    response_bytes: 812
+
+  redaction:
+    applied: true
+    rules:
+      - "drop-path-parameters"
+      - "pseudonymize-consumer"