docs: platform fee — sharer earns 90% of consumption, 10% to platform (v1.7)

[argszero]

Summary

Update docs/user-stories.md from v1.6 to v1.7 per host rant 2026-08-14T21:57:52 (平台收益抽成 10% — 分享者收益扣除 10% 平台分成).

Changes:

1. Platform fee rule: platform takes 10% of sharer earnings — consumer pays N pts → sharer receives N × 90% (per point rules, 2 decimals), N × 10% to platform. Example: A consumes B's key for 10 pts → A pays 10, B earns 9, platform 1
2. Core mechanism table: new row 「平台抽成 Platform fee」; 「收益 Earnings」updated (sharer earns consumption × 90%)
3. US-11 / US-24 (sharer AC): earn transaction = consumption pts × 90% (10% platform cut deducted); wallet "收益点数 · 永久" note
4. New edge case E-16 (小数抽成精度): earn = consumption × 90% with 2-decimal rounding (3 pts → 2.7 pts to sharer, 0.3 to platform; 1 pt → 0.9); sum of platform fee + sharer earning always equals consumer payment (rounding absorbed platform-side)
5. Operator role (one line): the 10% platform fee is the operator's revenue source
6. Alignment table updated

Related Issue

No issue — host rant 2026-08-14T21:57:52 (argszero/aitokenpool).

Changes

• 功能/修复说明：platform fee 10% on sharer earnings (docs v1.7)
• 涉及配置/数据结构的改动已同步示例文件：N/A (docs only; UI prototype follows in a separate PR)

Tests

• cargo test 全部通过 (1 passed)
• cargo fmt --check 通过
• 新增/更新了单元测试（如适用）：N/A — docs-only change

Checklist

• 分支命名符合约定：docs/platform-fee
• Commit message 使用 Conventional Commits：docs: platform fee — sharer earns 90% of consumption, 10% to platform (v1.7)
• 单一职责，改动最小化