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/0005-revision-identity.md

@@ -0,0 +1,47 @@
+# ADR-0005 — Revision identity
+
+**Status:** accepted · **Date:** 2026-09-04
+
+## Context
+
+Blueprint §54 lists this as deliberately open: "Should revision numbering be
+global, per interface, semantic, or content-addressed?" It asks for the answer
+to come from implementation experience rather than premature standardization.
+
+An answer is nonetheless needed before anything can be published, so this ADR
+picks the smallest one that does not foreclose the others.
+
+## Decision
+
+Two identifiers, with different jobs.
+
+**A human-facing revision id, sequential per interface.** `R-000221` with a
+`revision_number` of 22, both scoped to one interface. This is what appears in
+audit trails, CLI output and conversation.
+
+**A content address for every artifact the revision names.** The contract and
+the implementation each carry a `sha256:` digest. A revision is reproducibly
+associated with its artifacts through those digests, per Blueprint §25.
+
+Identity is therefore human-sequential; equality is content-addressed.
+
+## Rationale
+
+Global numbering was rejected: it couples unrelated interfaces and makes the
+number meaningless as a lineage signal.
+
+Purely content-addressed identity was rejected as the primary handle. Digests
+are correct but unreadable, and Blueprint §16 of the schema spec is explicit
+that human-readable prefixes are recommended for operational tooling. A framework
+whose central concept cannot be said out loud will not get used carefully.
+
+Semantic versioning was rejected because compatibility is already declared
+separately, in `compatibility.class`. Encoding it a second time in the
+identifier invites the two to disagree.
+
+## What stays open
+
+Whether `revision_number` should be dense (no gaps) is unresolved. Failed
+candidates currently consume a number. That is defensible — the attempt is part
+of the history — but it may prove noisy. Revisit after the first real interface
+has produced enough failed candidates to tell.