Finalize conversation and execution group documentation

coordinate.go

@@ -43,11 +43,20 @@ type CoordinationResult struct {
 }
 
 // Coordinate advances one branch from Start using ordinary runs. Next is called
-// initially even for a zero budget and receives each completed result once.
+// initially even for a zero budget and receives each completed result once,
+// including the final permitted response. The initial exchange is not itself a
+// newly delivered result. Each started continuation consumes one turn; retries
+// and structural repairs do not. Callback work runs after admission is released.
 // A nil request stops; requesting a run without remaining budget exhausts it.
 // Callback and execution errors retain their identities. Cancellation observed
 // before delivery returns the single completed response as Undelivered;
-// cancellation after callback invocation does not replay that response.
+// cancellation after callback invocation does not replay that response. Context
+// cancellation takes precedence over a callback's returned action or error.
+// Start must be a captured exchange, Next non-nil, and AdditionalTurns
+// nonnegative; invalid values match ErrInvalidRequest. A nil engine returns
+// ErrInvalidConfig. The selected request is copied and its ContinueFrom is
+// replaced by the coordinator's current exchange. The request observer runs
+// before the coordinator observer for each continuation.
 func (e *Engine) Coordinate(ctx context.Context, req CoordinateRequest) (CoordinationResult, error) {
 	if e == nil || e.internal == nil {
 		return CoordinationResult{}, fmt.Errorf("%w: engine is nil", ErrInvalidConfig)