docs: document relative_map workflow — start, path generation, and stop

[Copilot]

The modules/world_model/relative_map README only listed inputs/outputs with no explanation of how the module actually operates. This adds a full Workflow section covering every major stage.

Changes to README.md

• Module Start — 100 ms TimerComponent; four topic readers (perception_obstacles, chassis, localization/pose, navigation); config loading via relative_map_config.pb.txt
• Periodic Processing — Proc → Process → CreateMapFromNavigationLane call chain under mutex; publishes MapMsg to /apollo/relative_map
• Path Generation (GeneratePath) — two-tier priority cascade:
  • Offline-navigation mode (OFFLINE_GENERATED): ENU→FLU coordinate conversion, ~250 m segment selection, left-to-right lane sorting, lane-marker merge (90 % nav line + 10 % marker), perceived/default width assignment, cyclic-route stitching
  • Perception-only fallback: cubic-polynomial centerline from left/right lane-marker coefficients (y = c0 + c1·x + c2·x² + c3·x³), speed-proportional path length (150–250 m)
• Map Creation (CreateMap) — hdmap::Lane entries with shared boundaries between adjacent lanes, Road wrapper, forward neighbour IDs
• Module Stop — Stop() log; no cleanup needed

Original prompt

modules/world_model/relative_map的工作流程是什么，什么时候开始，什么时候结束，中间是如何生成路径的？

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