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/0003-wire-contract-as-boundary.md

@@ -0,0 +1,43 @@
+# ADR-0003 — The wire contract is the boundary
+
+**Status:** accepted · **Date:** 2026-09-04
+
+## Context
+
+ADR-0002 puts fluid-core in a separate process from everything it serves.
+Something has to define how those processes agree.
+
+## Decision
+
+The boundary is a set of on-the-wire artifacts, not a Go API:
+
+1. the revision descriptor (Blueprint §36);
+2. the routing policy (Blueprint §17);
+3. the telemetry envelope;
+4. the control APIs (Blueprint §44).
+
+These live in `schemas/` as JSON Schema. Go types in `internal/contract` are
+**generated** from those schemas and are never hand-written.
+
+## Rationale
+
+Field names and semantics are the interop surface — the schema specification
+says so directly in its §1. Generating from the schema means the specification
+cannot drift from the implementation without failing the build.
+
+It also makes the Go choice reversible where it matters. A future high-throughput
+gateway in another language, a Python Daimon, a TypeScript adapter: none of them
+need anything from this repository except the schemas.
+
+## Enforcement
+
+- `make check-generated` fails CI when `internal/contract` is stale.
+- `conformance/validate_schemas.py` validates fixtures transcribed from the
+  spec's own worked examples, so a spec change that the schemas miss fails too.
+- The Go round-trip test decodes those fixtures with `DisallowUnknownFields`,
+  catching any field the generated types have no home for.
+
+## Consequence
+
+Any change that would leak a Go type across this boundary is a design failure,
+not a convenience. If a consumer needs something, it goes in a schema first.