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

internal/contract/routing_policy_gen.go

@@ -0,0 +1,82 @@
+// Code generated by tools/schemagen. DO NOT EDIT.
+// Source: schemas/. Regenerate with `make generate`.
+
+package contract
+
+// What keeps a long-lived consumer on one side of an experiment.
+type RoutingPolicyRulesItemStickyBy string
+
+const (
+	RoutingPolicyRulesItemStickyByConsumerID    RoutingPolicyRulesItemStickyBy = "consumer_id"
+	RoutingPolicyRulesItemStickyByTenant        RoutingPolicyRulesItemStickyBy = "tenant"
+	RoutingPolicyRulesItemStickyByCorrelationID RoutingPolicyRulesItemStickyBy = "correlation_id"
+	RoutingPolicyRulesItemStickyByNone          RoutingPolicyRulesItemStickyBy = "none"
+)
+
+// Valid reports whether v is a defined RoutingPolicyRulesItemStickyBy.
+func (v RoutingPolicyRulesItemStickyBy) Valid() bool {
+	switch v {
+	case RoutingPolicyRulesItemStickyByConsumerID, RoutingPolicyRulesItemStickyByTenant, RoutingPolicyRulesItemStickyByCorrelationID, RoutingPolicyRulesItemStickyByNone:
+		return true
+	}
+	return false
+}
+
+type RoutingPolicyRulesItem struct {
+	// Revision id to traffic share. Shares must sum to 1. Assignment of a given
+	// consumer must be stable across requests.
+	Allocation map[string]UnitInterval `json:"allocation" yaml:"allocation"`
+	Cohort     *CohortID               `json:"cohort,omitempty" yaml:"cohort,omitempty"`
+	Experiment *ExperimentID           `json:"experiment,omitempty" yaml:"experiment,omitempty"`
+
+	// What keeps a long-lived consumer on one side of an experiment.
+	StickyBy *RoutingPolicyRulesItemStickyBy `json:"sticky_by,omitempty" yaml:"sticky_by,omitempty"`
+	Tenant   string                          `json:"tenant,omitempty" yaml:"tenant,omitempty"`
+}
+
+type RoutingPolicySignatureAlgorithm string
+
+const (
+	RoutingPolicySignatureAlgorithmEd25519 RoutingPolicySignatureAlgorithm = "ed25519"
+)
+
+// Valid reports whether v is a defined RoutingPolicySignatureAlgorithm.
+func (v RoutingPolicySignatureAlgorithm) Valid() bool {
+	switch v {
+	case RoutingPolicySignatureAlgorithmEd25519:
+		return true
+	}
+	return false
+}
+
+type RoutingPolicySignature struct {
+	Algorithm RoutingPolicySignatureAlgorithm `json:"algorithm" yaml:"algorithm"`
+	KeyID     string                          `json:"key_id" yaml:"key_id"`
+	Value     string                          `json:"value" yaml:"value"`
+}
+
+type RoutingPolicy struct {
+	// Where a request lands when no earlier resolution step matched.
+	DefaultRevision RevisionID `json:"default_revision" yaml:"default_revision"`
+
+	// Monotonic. The router ignores a policy older than the one it holds.
+	Generation int64       `json:"generation" yaml:"generation"`
+	ID         string      `json:"id,omitempty" yaml:"id,omitempty"`
+	Interface  InterfaceID `json:"interface" yaml:"interface"`
+	IssuedAt   *Timestamp  `json:"issued_at,omitempty" yaml:"issued_at,omitempty"`
+	IssuedBy   *Actor      `json:"issued_by,omitempty" yaml:"issued_by,omitempty"`
+
+	// Evaluated in order; the first matching rule wins. Matching must be a pure
+	// function of the request and its cohort assignment.
+	Rules         []RoutingPolicyRulesItem `json:"rules" yaml:"rules"`
+	SchemaVersion SchemaVersion            `json:"schema_version" yaml:"schema_version"`
+	Signature     *RoutingPolicySignature  `json:"signature,omitempty" yaml:"signature,omitempty"`
+}
+
+// Deterministic routing written by the experiment controller and consumed by the
+// revision router. The controller never touches traffic itself; this separation
+// keeps experimental intent out of the runtime decision mechanism
+// (ArchitectureBlueprint.md section 17).
+type RoutingPolicyDocument struct {
+	RoutingPolicy RoutingPolicy `json:"routing_policy" yaml:"routing_policy"`
+}