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

bindings/node/index.d.ts

@@ -4544,6 +4544,51 @@ export declare class CalendarSpread {
   isReady(): boolean
   warmupPeriod(): number
 }
+export type EstimatedLeverageRatioNode = EstimatedLeverageRatio
+export declare class EstimatedLeverageRatio {
+  constructor()
+  update(openInterest: number, longSize: number, shortSize: number): number | null
+  batch(openInterest: Array<number>, longSize: Array<number>, shortSize: Array<number>): Array<number>
+  reset(): void
+  isReady(): boolean
+  warmupPeriod(): number
+}
+export type OiToVolumeRatioNode = OiToVolumeRatio
+export declare class OiToVolumeRatio {
+  constructor()
+  update(openInterest: number, takerBuyVolume: number, takerSellVolume: number): number | null
+  batch(openInterest: Array<number>, takerBuyVolume: Array<number>, takerSellVolume: Array<number>): Array<number>
+  reset(): void
+  isReady(): boolean
+  warmupPeriod(): number
+}
+export type PerpetualPremiumIndexNode = PerpetualPremiumIndex
+export declare class PerpetualPremiumIndex {
+  constructor()
+  update(markPrice: number, indexPrice: number): number | null
+  batch(markPrice: Array<number>, indexPrice: Array<number>): Array<number>
+  reset(): void
+  isReady(): boolean
+  warmupPeriod(): number
+}
+export type FundingImpliedAprNode = FundingImpliedApr
+export declare class FundingImpliedApr {
+  constructor(intervalsPerYear: number)
+  update(fundingRate: number): number | null
+  batch(fundingRate: Array<number>): Array<number>
+  reset(): void
+  isReady(): boolean
+  warmupPeriod(): number
+}
+export type OpenInterestMomentumNode = OpenInterestMomentum
+export declare class OpenInterestMomentum {
+  constructor(period: number)
+  update(openInterest: number): number | null
+  batch(openInterest: Array<number>): Array<number>
+  reset(): void
+  isReady(): boolean
+  warmupPeriod(): number
+}
 export type AdvanceDeclineNode = AdvanceDecline
 export declare class AdvanceDecline {
   constructor()