feat: add built-in strategy templates and update docs

- Add eqlib/strategies/ package with 3 ready-to-use strategy templates:
  - momentum_rotation: multi-factor scoring with daily rebalancing
  - ma_crossover: classic dual-MA golden/death cross strategy
  - grid_trading: range-bound grid trading strategy
- Export strategies submodule from eqlib.__init__
- Update examples/README.md with bonus Web Studio strategy section
- Update README.md and README_zh.md example counts

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>