Allow multiple HandlerOrder attributes on a single handler class

[Skymly]

Summary

Enable one handler class to participate in multiple generated pipelines by applying multiple [HandlerOrder] / [HandlerOrder<TContext>] attributes (different context types and/or orders).

Motivation

Strategy and Factory already use AllowMultiple = true. Handler is the outlier: shared cross-cutting handlers (logging, telemetry) should register on more than one IHandler<TContext> pipeline without duplicating types.

Acceptance criteria

• HandlerOrderAttribute and HandlerOrderAttribute<TContext> use AllowMultiple = true
• HandlerOrderGenerator emits one registration per attribute instance; pipelines are still grouped by context type
• Existing duplicate-order (DP005), contract mismatch (DP008), and parameterless constructor (DP009) diagnostics unchanged
• Generator Verify test + integration test for one class with two contexts
• Document in docs/ChainOfResponsibility.md; update docs/ROADMAP.md

Module

DesignPatterns/ (attributes) + DesignPatterns.SourceGenerators/ + tests/