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

Makefile

@@ -0,0 +1,54 @@
+# FLUID core — build, generation and conformance.
+#
+# The record types in internal/contract are generated from schemas/. Never edit
+# them by hand: run `make generate`.
+
+GO      ?= go
+PYTHON  ?= python3
+PKGS    := ./...
+
+.PHONY: all
+all: generate build vet test validate
+
+.PHONY: generate
+generate: ## Regenerate Go types from schemas/
+	$(GO) run ./tools/schemagen
+	gofmt -w internal/contract
+
+.PHONY: build
+build:
+	$(GO) build $(PKGS)
+
+.PHONY: vet
+vet:
+	$(GO) vet $(PKGS)
+
+.PHONY: test
+test: validate
+	$(GO) test $(PKGS)
+
+.PHONY: validate
+validate: ## Validate spec-derived fixtures against the schemas
+	$(PYTHON) conformance/validate_schemas.py
+
+.PHONY: check-generated
+check-generated: ## Fail if generated code is stale relative to schemas/
+	@$(MAKE) --no-print-directory generate
+	@if ! git diff --quiet -- internal/contract; then \
+		echo "internal/contract is stale — run 'make generate' and commit the result"; \
+		git --no-pager diff --stat -- internal/contract; \
+		exit 1; \
+	fi
+
+.PHONY: conformance
+conformance: ## Full conformance suite (grows through FLUID-WP-0007)
+	$(GO) test ./conformance/... $(PKGS)
+
+.PHONY: clean
+clean:
+	rm -rf bin internal/contract/*.broken
+
+.PHONY: help
+help:
+	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | \
+		awk 'BEGIN {FS = ":.*?## "}; {printf "  \033[36m%-18s\033[0m %s\n", $$1, $$2}'