Add revision registry and deterministic resolver

The registry is a cached snapshot rather than a control-plane client, so
the data plane keeps serving when the control plane dies (Blueprint
34.6). Routing policy generations are monotonic: a delayed older policy
is refused rather than silently rolling back an in-flight experiment's
allocation.

The resolver implements the Blueprint 5.2 precedence chain and records
why each revision was chosen. Experiment allocation is a deterministic
function of a sticky key namespaced by experiment id, so a consumer
stays in one arm for the experiment's duration and does not land in the
same arm of every concurrent experiment.

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/runtime/resolver.go

@@ -0,0 +1,212 @@
+package runtime
+
+import (
+	"fmt"
+	"hash/fnv"
+	"sort"
+
+	"github.com/tegwick/fluid-core/internal/contract"
+)
+
+// Request is the subset of an inbound request that revision resolution may
+// consider. Nothing else is allowed to influence the decision: resolution must
+// be a pure function of these fields and the loaded policy, or it stops being
+// auditable (ArchitectureBlueprint.md section 5.2).
+type Request struct {
+	// ExplicitRevision is a revision the consumer asked for by name.
+	ExplicitRevision contract.RevisionID
+	// BoundRevision comes from a client contract binding.
+	BoundRevision contract.RevisionID
+	// Cohort is the consumer's cohort assignment.
+	Cohort contract.CohortID
+	// Tenant identifies the calling tenant, where the interface is multi-tenant.
+	Tenant string
+	// ConsumerRef is the pseudonymous, stable consumer identity used to keep a
+	// long-lived consumer on one side of an experiment.
+	ConsumerRef string
+	// CorrelationID ties this request to its telemetry.
+	CorrelationID string
+}
+
+// Resolution is the outcome of revision resolution, including why.
+//
+// The reason is not decoration. Blueprint section 5.2 requires resolution to be
+// auditable, and "which revision served this request" is unanswerable later
+// without recording how it was chosen.
+type Resolution struct {
+	Revision   contract.RevisionID
+	Reason     contract.FluidTelemetryResolutionReason
+	Experiment *contract.ExperimentID
+	// PolicyGeneration records which policy produced this decision.
+	PolicyGeneration int64
+}
+
+// Resolver implements the deterministic precedence chain.
+type Resolver struct {
+	registry *Registry
+	// allowExplicit controls whether consumers may pin a revision by name. Some
+	// interfaces want this for migration testing; others must not expose it.
+	allowExplicit bool
+}
+
+// NewResolver returns a resolver over reg.
+func NewResolver(reg *Registry, allowExplicit bool) *Resolver {
+	return &Resolver{registry: reg, allowExplicit: allowExplicit}
+}
+
+// Resolve selects the revision that will serve req.
+//
+// The order is fixed by ArchitectureBlueprint.md section 5.2:
+//
+//	explicit revision -> bound client contract -> experiment assignment -> stable default
+//
+// Each step is skipped rather than failed when the candidate is not routable,
+// so a retired pin or an ineligible cohort degrades to the default instead of
+// erroring the request.
+func (r *Resolver) Resolve(req Request) (Resolution, error) {
+	policy, err := r.registry.Policy()
+	if err != nil {
+		return Resolution{}, err
+	}
+
+	if r.allowExplicit && req.ExplicitRevision != "" {
+		if err := r.registry.CheckRoutable(req.ExplicitRevision, req.Cohort); err != nil {
+			// An explicit request for something unroutable is a consumer error
+			// worth surfacing, not something to silently reinterpret.
+			return Resolution{}, fmt.Errorf("explicit revision %s: %w", req.ExplicitRevision, err)
+		}
+		return Resolution{
+			Revision:         req.ExplicitRevision,
+			Reason:           contract.FluidTelemetryResolutionReasonExplicitRevision,
+			PolicyGeneration: policy.Generation,
+		}, nil
+	}
+
+	if req.BoundRevision != "" {
+		if err := r.registry.CheckRoutable(req.BoundRevision, req.Cohort); err == nil {
+			return Resolution{
+				Revision:         req.BoundRevision,
+				Reason:           contract.FluidTelemetryResolutionReasonBoundContract,
+				PolicyGeneration: policy.Generation,
+			}, nil
+		}
+	}
+
+	if rule, ok := matchRule(policy.Rules, req); ok {
+		chosen, ok := allocate(rule, req, policy.DefaultRevision)
+		if ok {
+			if err := r.registry.CheckRoutable(chosen, req.Cohort); err == nil {
+				reason := contract.FluidTelemetryResolutionReasonCohortRule
+				if rule.Experiment != nil {
+					reason = contract.FluidTelemetryResolutionReasonExperimentAssignment
+				}
+				return Resolution{
+					Revision:         chosen,
+					Reason:           reason,
+					Experiment:       rule.Experiment,
+					PolicyGeneration: policy.Generation,
+				}, nil
+			}
+		}
+	}
+
+	if err := r.registry.CheckRoutable(policy.DefaultRevision, req.Cohort); err != nil {
+		return Resolution{}, fmt.Errorf("default revision %s: %w", policy.DefaultRevision, err)
+	}
+	return Resolution{
+		Revision:         policy.DefaultRevision,
+		Reason:           contract.FluidTelemetryResolutionReasonStableDefault,
+		PolicyGeneration: policy.Generation,
+	}, nil
+}
+
+// matchRule returns the first rule matching the request. Rules are evaluated in
+// document order and the first match wins, so policy authors control precedence
+// by ordering rather than by scoring.
+func matchRule(rules []contract.RoutingPolicyRulesItem, req Request) (contract.RoutingPolicyRulesItem, bool) {
+	for _, rule := range rules {
+		if rule.Cohort != nil && *rule.Cohort != req.Cohort {
+			continue
+		}
+		if rule.Tenant != "" && rule.Tenant != req.Tenant {
+			continue
+		}
+		return rule, true
+	}
+	return contract.RoutingPolicyRulesItem{}, false
+}
+
+// allocate picks a revision from a rule's traffic shares.
+//
+// Assignment is a deterministic function of the sticky key, so a given consumer
+// lands on the same side of an experiment for its whole duration. Random
+// per-request assignment would make within-consumer comparisons meaningless and
+// would let a client observe both revisions at once.
+func allocate(rule contract.RoutingPolicyRulesItem, req Request, fallback contract.RevisionID) (contract.RevisionID, bool) {
+	if len(rule.Allocation) == 0 {
+		return "", false
+	}
+
+	// Sorting makes the traversal order independent of Go's map iteration, which
+	// is what turns a hash bucket into a stable assignment.
+	ids := make([]string, 0, len(rule.Allocation))
+	var total float64
+	for id, share := range rule.Allocation {
+		ids = append(ids, id)
+		total += float64(share)
+	}
+	sort.Strings(ids)
+
+	if total <= 0 {
+		return "", false
+	}
+
+	key := stickyKey(rule, req)
+	position := bucket(key) * total
+
+	var cumulative float64
+	for _, id := range ids {
+		cumulative += float64(rule.Allocation[id])
+		if position < cumulative {
+			return contract.RevisionID(id), true
+		}
+	}
+	// Floating-point drift at the top of the range.
+	return contract.RevisionID(ids[len(ids)-1]), true
+}
+
+// stickyKey chooses what keeps a consumer on one side of an experiment.
+func stickyKey(rule contract.RoutingPolicyRulesItem, req Request) string {
+	mode := contract.RoutingPolicyRulesItemStickyByConsumerID
+	if rule.StickyBy != nil {
+		mode = *rule.StickyBy
+	}
+
+	var subject string
+	switch mode {
+	case contract.RoutingPolicyRulesItemStickyByTenant:
+		subject = req.Tenant
+	case contract.RoutingPolicyRulesItemStickyByCorrelationID:
+		subject = req.CorrelationID
+	case contract.RoutingPolicyRulesItemStickyByNone:
+		subject = req.CorrelationID
+	default:
+		subject = req.ConsumerRef
+	}
+
+	// Namespacing by experiment stops one consumer from landing in the same
+	// arm of every concurrent experiment, which would confound their results.
+	if rule.Experiment != nil {
+		return string(*rule.Experiment) + "\x00" + subject
+	}
+	return subject
+}
+
+// bucket maps a key into [0, 1).
+func bucket(key string) float64 {
+	h := fnv.New64a()
+	_, _ = h.Write([]byte(key))
+	// 53 bits keeps the result exactly representable as a float64.
+	const mask = 1<<53 - 1
+	return float64(h.Sum64()&mask) / float64(mask+1)
+}