feat(pipeline): serial + parallel plugin execution with declared ordering

[huang195]

From the AuthBridge roadmap (Deep Dive → Next Steps · Core functions).

Support parallel execution of independent plugins and serial ordering via declared dependencies (Requires / RequiresAny / After), validated at pipeline.New (build-time graph check rather than runtime surprise).

Related: #398 (arbitration of overlapping plugin responsibilities). Tracking issue for the execution model.