docs: drop process artefacts, describe the project on its own terms

[HowieMen]

Reader-facing artefacts (README, contract) and in-code comments used to expose the project's development history — task numbers, hotfix versions, audit trail — alongside the actual content. A new reader does not care which TODO.md row a behaviour came from; they want to know what the project is, how to use it, and what its limits are.

This commit strips the process labels so the project reads as a normal release:

• Delete CHANGELOG.md and TODO.md (both dev-process artefacts).
• Rename eight test_task{N}_*.py files to describe the area they cover (e.g. test_strategy_isolation.py, test_metrics_baseline.py).
• Remove ~140 Task X: / task X: / TODO.md / CHANGELOG.md references from src/ and tests/. Keep the design intent, drop the development label.
• Rewrite the README's "项目状态" section as a "当前能力（按模块）"
  • "已知限制" pair: the project described by what it does and what it does not, not by which task made it so.
• Compress mvp-contract.md §9 "修订记录" from a per-task changelog into a single, deduplicated record of contract changes; remove the "对应 TODO.md 任务 1" header.

No API or runtime behavior change. 668 unit tests + 4 real-data integration tests stay green; black --check clean; python -m build produces a v0.1.4 sdist + wheel.