feat(derivatives): leverage, OI/volume, perpetual premium, funding APR, OI momentum (B16) (#214)
## B16 Derivatives — five new indicators (488 → 493)
All consume a `DerivativesTick` and emit `f64`:
| Indicator | Reads | Formula |
|-----------|-------|---------|
| `EstimatedLeverageRatio` | open_interest, long_size, short_size | `OI / (long + short)` |
| `OiToVolumeRatio` | open_interest, taker_buy_volume, taker_sell_volume | `OI / (buy + sell)` |
| `PerpetualPremiumIndex` | mark_price, index_price | `(mark − index) / index` |
| `FundingImpliedApr` | funding_rate | `rate × intervals_per_year` |
| `OpenInterestMomentum` | open_interest | `100 · (OI_t − OI_{t−period}) / OI_{t−period}` |
### Wiring
- Core structs + full unit tests (incl. zero-denominator branches).
- Hand-written Python/Node/WASM tick bindings; two new tick helpers (`deriv_oi_long_short`, `deriv_oi_taker`).
- Fuzz drives in `indicator_update_derivatives.rs`; dedicated reference + streaming-vs-batch tests (Python + Node).
- README counter + `docs/README.md` + `FAMILIES` assert bumped to 493.
### Verify (local, all green)
- `cargo test -p wickra-core --lib`: 4028 · `--doc`: 443
- clippy workspace: clean
- node: 563 · pytest: 928
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