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

docs/adr/0004-evidence-store-sqlite-to-postgres.md

@@ -0,0 +1,37 @@
+# ADR-0004 — Append-only evidence store, SQLite then Postgres
+
+**Status:** accepted · **Date:** 2026-09-04
+
+## Context
+
+Blueprint §26 requires an evidence store favouring append-only history, with
+mutable summaries derived from immutable events. §45 suggests a relational
+starting model and explicitly argues it is simpler than a graph database for a
+first implementation.
+
+## Decision
+
+One relational schema, two backends. SQLite for development, single-node
+deployments and the CI conformance suite; PostgreSQL for anything shared. The
+event table is append-only: no `UPDATE`, no `DELETE`, enforced by trigger rather
+than by convention.
+
+Summary tables are derived views, rebuildable from events at any time.
+
+## Rationale
+
+The first real workload — publishing hall-of-helix entries to a Telegram channel
+— produces a handful of events per day. Requiring Postgres to run the framework
+at that scale would be an operational tax with no return.
+
+Keeping one schema across both means the CI suite exercises the same statements
+production runs, which is where divergence usually hides.
+
+## Consequences
+
+- Portable SQL only; no backend-specific features in the core path.
+- Blueprint invariant 8 (every promotion is auditable) is a storage property,
+  not an application convention: rewriting history has to be blocked at the
+  database.
+- Evidence-store failure must not stop the data plane (Blueprint §34.6). The
+  gateway serves from cached published configuration and buffers telemetry.