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

.github/workflows/ci.yml

@@ -0,0 +1,42 @@
+name: ci
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: '1.22'
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
+      - name: Install schema validation dependencies
+        run: pip install --quiet jsonschema pyyaml
+
+      # The spec-derived fixtures are validated first: if the specification and
+      # the schemas have diverged, nothing downstream is worth building.
+      - name: Validate schemas against spec fixtures
+        run: make validate
+
+      # Generated types must match schemas/. A stale internal/contract means a
+      # schema change landed without regeneration.
+      - name: Check generated code is current
+        run: make check-generated
+
+      - name: Build
+        run: make build
+
+      - name: Vet
+        run: make vet
+
+      - name: Test
+        run: make test