feat: add practical mainland payment knowledge base

[Fangx-AI]

What changed

• Added 11 structured payment platform profiles covering official direct payment, service providers, and creator platforms in mainland China.
• Added 6 decision scenarios, a payment router, source policy, and a 13-item research backlog.
• Updated fxskill to follow “资料库深、回答浅”: one clear conclusion, 2–4 decision-relevant facts, and at most 3 ordered actions.
• Added a payment knowledge-base validator and behavior/regression tests.

Why

Beginner payment questions need concrete platform conditions, fees, settlement paths, and practical next steps without turning every answer into a long manual. This establishes an evidence-bounded foundation that can be expanded gradually.

User impact

Payment answers can now route by seller identity and payment model, compare named platforms without inventing certainty, and fall back to a plain-language official verification path when evidence is incomplete.

Validation

• py -3 -m unittest -v tests.fxskill.test_payment_kb — 97 passed
• py -3 scripts/validate_payment_kb.py — 11 profiles validated
• quick_validate.py skills/fxskill — Skill is valid
• Full-branch review — Critical 0, Important 0