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/0001-go-as-implementation-language.md

@@ -0,0 +1,46 @@
+# ADR-0001 — Go as the implementation language
+
+**Status:** accepted · **Date:** 2026-09-04
+
+## Context
+
+fluid-core had no implementation. Three stacks were credible: Go, TypeScript
+and Python. The specification (`ArchitectureBlueprint.md` §42–43) points at a
+long-running gateway, a router, an append-only store and, later, a Kubernetes
+controller with custom resources.
+
+Python is the obvious first reach — pydantic maps almost literally onto the
+record schemas, and it matches existing tooling in sibling repositories.
+TypeScript is attractive because the first consumers are platform SDKs with
+strong TypeScript clients.
+
+## Decision
+
+fluid-core is written in Go.
+
+## Rationale
+
+The always-on component is infrastructure: proxy, router, registry, evidence
+store. That is Go's centre of mass. A single static binary is also the most
+credible form of "drop this in front of any API" — the promise ADR-0002 makes.
+
+Blueprint §43 maps 1:1 onto controller-runtime, so the Kubernetes path stays
+open at no extra cost.
+
+Go is the lowest-entropy of the three to debug, which matters under a per-task
+budget policy.
+
+## What we gave up
+
+Record modeling is more verbose in Go than in Python. This is paid once, behind
+generated types (ADR-0003), rather than continuously.
+
+The generative Daimon (Blueprint Phase D) would be easier in Python. ADR-0002
+makes that a non-issue: the Daimon is a separate process speaking the wire
+contract, so it may be written in whatever suits it.
+
+## Consequences
+
+- No third-party Go modules are used where the standard library suffices; the
+  build stays offline-capable.
+- Python remains a build-time dependency for schema validation only.