feat: derivatives funding & open-interest indicators (part 1 of 3) (#126)

* feat(derivatives): DerivativesTick input type + InvalidDerivatives error

* feat(derivatives): FundingRate indicator (core)

* feat(derivatives): FundingRateMean indicator (core)

* feat(derivatives): FundingRateZScore indicator (core)

* feat(derivatives): FundingBasis indicator (core)

* feat(derivatives): OpenInterestDelta indicator (core)

* feat(derivatives): Python, Node and WASM bindings for funding & OI-delta indicators

* test(derivatives): Python and Node tests for funding & OI-delta indicators

* bench(derivatives): synthetic-tick bench + derivatives fuzz target

* docs(derivatives): README family row + counter 232->237, CHANGELOG entry

crates/wickra-core/src/error.rs

@@ -43,6 +43,15 @@ pub enum Error {
     /// non-finite price or negative size) was provided.
     #[error("invalid trade: {message}")]
     InvalidTrade { message: &'static str },
+
+    /// A derivatives tick whose components do not satisfy the tick invariants
+    /// (e.g. a non-positive price, a non-finite funding rate, or a negative
+    /// size/volume/liquidation) was provided. Derivatives ticks (funding /
+    /// open-interest / liquidation feeds) are a perpetual-futures input
+    /// distinct from candles, order books and trades, so they surface as their
+    /// own variant.
+    #[error("invalid derivatives tick: {message}")]
+    InvalidDerivatives { message: &'static str },
 }
 
 /// Convenience alias for `Result<T, wickra_core::Error>`.