fix(trace): an action names the generator that produced it

The setup exclusion landed for the model arm only, because only a model
pick stamped a source. A seeded run returned setup's action through the
same entry with no marker, so its denominator still counted the login
while the model arm's did not, and the two are compared.

serializeAction names setup and seeded on the wire, so both arms are
counted by one rule. An already-recorded trace names nothing and keeps
exactly the count it was reported with; unattributed_actions counts those
steps so the old denominator cannot pass as the new one. TraceVersion is
deliberately unbumped: oracle-reduction refuses a differing version, and
a bump would make all 169 recorded runs unreplayable.

internal/trace/writer.go

@@ -109,6 +109,16 @@ type Metrics struct {
 	TotalMemoryBytes int64   `json:"total_memory_bytes,omitempty"`
 }
 
+// The three producers an action can come from. The spec's setup drives the app
+// into position and explores nothing, so a per-action rate divides by the other
+// two; an action recorded before this distinction existed carries none of them
+// and cannot be attributed after the fact.
+const (
+	ActionSourceSetup  = "setup"
+	ActionSourceSeeded = "seeded"
+	ActionSourceModel  = "llm"
+)
+
 type Action struct {
 	Kind           string        `json:"kind"`
 	X              int           `json:"x,omitempty"`
@@ -123,8 +133,8 @@ type Action struct {
 	Selector       string        `json:"selector,omitempty"`
 	ResolvedBounds *BoundsRecord `json:"resolved_bounds,omitempty"`
 	TapPoint       *PointRecord  `json:"tap_point,omitempty"`
-	// Source names the backend that chose this action: "llm" when the LLM
-	// action backend selected it, empty for the seeded picker. LLMReasoning is
+	// Source names which of the three producers chose this action, and is empty
+	// only on a trace recorded before actions named themselves. LLMReasoning is
 	// the model's short rationale, shown by the replay UI to explain the pick.
 	Source       string `json:"source,omitempty"`
 	LLMReasoning string `json:"llm_reasoning,omitempty"`