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/pressure.json

@@ -0,0 +1,25 @@
+{
+  "fluid_pressure": {
+    "affected_cohorts": [
+      "partner-integrations"
+    ],
+    "class": "successful_but_inefficient_usage",
+    "evidence_refs": [
+      "telemetry:query-pattern-712"
+    ],
+    "first_seen": "2026-08-15T10:11:00Z",
+    "frequency": {
+      "independent_consumers": 83,
+      "observations": 1740
+    },
+    "id": "P-1831",
+    "interface_id": "customer-finance-api",
+    "last_seen": "2026-09-01T12:15:00Z",
+    "linked_hypotheses": [
+      "H-000184"
+    ],
+    "schema_version": "0.1",
+    "status": "OPEN",
+    "summary": "Consumers repeatedly retrieve complete invoice collections to determine the latest invoice.\n"
+  }
+}