Detach prepared execution ownership from runners

docs/roadmap/implementation.md

@@ -266,6 +266,10 @@ same-engine views, one-shot/concurrent claim/discard and details after use.
 Session Unicode/trim/content/wire/hash tests stay unchanged in meaning. Reference
 analysis, not a GC-timing assertion, establishes that the token retains no runner.
 
+Completion: prepared handles retain only a private nonzero-sized runner identity
+token, including after use or discard; accepted session strings own their
+storage. Focused prepared/session race tests pass on Go 1.27.3.
+
 ## Stage 10: Declare Validation Preparation As The Internal Dependency
 
 **Catalog:** 21, interface part. **Files:** validator constructors, engine Options,

internal/domain/session.go

@@ -18,5 +18,5 @@ func NormalizeSessionID(raw string) (string, error) {
 	if length := utf8.RuneCountInString(normalized); length > SessionIDMaxLength {
 		return "", fmt.Errorf("session_id length %d exceeds maximum %d", length, SessionIDMaxLength)
 	}
-	return normalized, nil
+	return strings.Clone(normalized), nil
 }

internal/usecase/prepared_execution.go

@@ -22,7 +22,7 @@ const (
 
 // PreparedExecution owns one frozen, single-use runner execution.
 type PreparedExecution struct {
-	owner   *Runner
+	owner   *runnerIdentity
 	mu      sync.Mutex
 	state   preparedExecutionState
 	details *domain.PreparedRun
@@ -63,7 +63,7 @@ func (r *Runner) PrepareExecution(ctx context.Context, req domain.RunRequest) (*
 	}
 
 	return &PreparedExecution{
-		owner:   r,
+		owner:   r.identity,
 		state:   preparedExecutionReady,
 		details: details,
 		payload: &preparedExecutionPayload{
@@ -170,7 +170,7 @@ func (r *Runner) RunPrepared(ctx context.Context, prepared *PreparedExecution) (
 }
 
 func (p *PreparedExecution) claim(owner *Runner) (*preparedExecutionPayload, error) {
-	if p == nil || owner == nil || p.owner != owner {
+	if p == nil || owner == nil || owner.identity == nil || p.owner != owner.identity {
 		return nil, fmt.Errorf("%w: prepared execution does not belong to this runner", ErrInvalidRequest)
 	}
 

internal/usecase/prepared_execution_test.go

@@ -600,6 +600,9 @@ func TestRunnerPreparedExecutionOwnershipUseAndDiscard(t *testing.T) {
 	}
 
 	owner := newRunner()
+	if other := newRunner(); owner.identity == nil || owner.identity == other.identity {
+		t.Fatal("constructed runners must have independent identity tokens")
+	}
 	prepared, err := owner.PrepareExecution(context.Background(), request)
 	if err != nil {
 		t.Fatalf("prepare execution: %v", err)
@@ -616,6 +619,9 @@ func TestRunnerPreparedExecutionOwnershipUseAndDiscard(t *testing.T) {
 	if prepared.Details() == nil {
 		t.Fatal("details unavailable after execution")
 	}
+	if prepared.owner != owner.identity || prepared.payload != nil {
+		t.Fatal("claimed handle retained payload or lost owner identity")
+	}
 
 	discarded, err := owner.PrepareExecution(context.Background(), request)
 	if err != nil {
@@ -623,6 +629,9 @@ func TestRunnerPreparedExecutionOwnershipUseAndDiscard(t *testing.T) {
 	}
 	discarded.Discard()
 	discarded.Discard()
+	if discarded.owner != owner.identity || discarded.payload != nil {
+		t.Fatal("discarded handle retained payload or lost owner identity")
+	}
 	if _, err := owner.RunPrepared(context.Background(), discarded); !errors.Is(err, ErrInvalidRequest) {
 		t.Fatalf("discarded execution error=%v, want ErrInvalidRequest", err)
 	}

internal/usecase/runner.go

@@ -43,6 +43,7 @@ var (
 
 // Runner executes the Promptkit core use case.
 type Runner struct {
+	identity                *runnerIdentity
 	promptDefs              promptdef.Repository
 	profiles                profile.Repository
 	artifacts               artifact.Reader
@@ -54,6 +55,10 @@ type Runner struct {
 	defaultTransientRetries uint64
 }
 
+// A non-zero-sized token gives each constructed runner a distinct identity
+// without retaining its collaborators in prepared handles.
+type runnerIdentity struct{ marker byte }
+
 // RunAdmitter reserves capacity for one resolved backend run.
 type RunAdmitter interface {
 	Admit(context.Context, string) (func(), error)
@@ -113,6 +118,7 @@ func NewRunnerWithRepairer(
 		retries = defaultTransientRetries[0]
 	}
 	return &Runner{
+		identity:                &runnerIdentity{},
 		promptDefs:              promptDefs,
 		profiles:                profiles,
 		artifacts:               artifacts,