B5 volatility & bands batch (423 -> 429) (#189)

Adds six **Volatility & Bands** indicators (Part B5 of the expansion roadmap), 423 → 429.

| Indicator | Input → Output | Summary |
|-----------|----------------|---------|
| `EwmaVolatility` | `f64` → `f64` | RiskMetrics exponentially-weighted volatility (λ decay) |
| `Garch11` | `f64` → `f64` | GARCH(1,1) conditional volatility with a long-run-variance anchor |
| `BipowerVariation` | `f64` → `f64` | jump-robust realized bipower variation (π/2 · Σ\|rₜ\|\|rₜ₋₁\|) |
| `VolatilityRatio` | `Candle` → `f64` | Schwager's true range over the EMA of prior true ranges (>2 = wide-ranging day) |
| `VolatilityCone` | `Candle` → `VolatilityConeOutput` | current realized volatility within its min/median/max envelope + percentile |
| `VolatilityOfVolatility` | `f64` → `f64` | sample stddev of a rolling realized-volatility series |

### Notes
- Two B5 roadmap items were dropped as duplicates/by-construction: `RealizedVolatility` already ships (v0.5.4); `Downside Semi-Deviation` is internal to Sortino. `Bipower Variation` confirmed distinct from `JumpIndicator` (a ±1 flag, not a variance measure).
- `VolatilityRatio` implements the widely-charted EMA-of-true-range convention (denominator excludes the current bar so the 2.0 threshold means "twice typical"), distinct from the existing pairwise `variance_ratio`.
- `Garch11` mean-reverts to `ω/(1−β)` on a flat series (does not decay to 0 like EWMA) — pinned by a dedicated test.

### Coverage / verification
- Full core + Python/Node/WASM bindings, fuzz drivers (scalar + candle), registries, CHANGELOG, README + docs counter sync.
- 100% unit-test coverage per indicator (every branch).
- Green locally: `cargo clippy --workspace --all-targets --all-features -D warnings`, core lib (3479) + doc (387), node (504), python (830).

Deep-dive docs for all six are staged for `wickra-docs` and pushed after release (gated).

bindings/node/index.d.ts

@@ -5,6 +5,17 @@
 
 /** Library version (matches the Rust crate version). */
 export declare function version(): string
+/**
+ * Volatility-cone result: current realized volatility and its lookback
+ * envelope (min / median / max) plus the percentile rank of `current`.
+ */
+export interface VolatilityConeValue {
+  current: number
+  min: number
+  median: number
+  max: number
+  percentile: number
+}
 /** Lead/lag result: the offset that maximises correlation, and that correlation. */
 export interface LeadLagValue {
   /** Offset that maximises `|corr(a, b shifted)|`. Positive ⇒ `a` leads `b`. */
@@ -987,6 +998,51 @@ export declare class TsfOscillator {
   isReady(): boolean
   warmupPeriod(): number
 }
+export type BipowerVariationNode = BipowerVariation
+export declare class BipowerVariation {
+  constructor(period: number)
+  update(value: number): number | null
+  batch(prices: Array<number>): Array<number>
+  reset(): void
+  isReady(): boolean
+  warmupPeriod(): number
+}
+export type EwmaVolatilityNode = EwmaVolatility
+export declare class EwmaVolatility {
+  constructor(lambda: number)
+  update(value: number): number | null
+  batch(prices: Array<number>): Array<number>
+  reset(): void
+  isReady(): boolean
+  warmupPeriod(): number
+}
+export type Garch11Node = Garch11
+export declare class Garch11 {
+  constructor(omega: number, alpha: number, beta: number)
+  update(value: number): number | null
+  batch(prices: Array<number>): Array<number>
+  reset(): void
+  isReady(): boolean
+  warmupPeriod(): number
+}
+export type VolatilityOfVolatilityNode = VolatilityOfVolatility
+export declare class VolatilityOfVolatility {
+  constructor(volWindow: number, vovWindow: number)
+  update(value: number): number | null
+  batch(prices: Array<number>): Array<number>
+  reset(): void
+  isReady(): boolean
+  warmupPeriod(): number
+}
+export type VolatilityConeNode = VolatilityCone
+export declare class VolatilityCone {
+  constructor(window: number, lookback: number)
+  update(high: number, low: number, close: number): VolatilityConeValue | null
+  batch(high: Array<number>, low: Array<number>, close: Array<number>): Array<number>
+  reset(): void
+  isReady(): boolean
+  warmupPeriod(): number
+}
 export type JumpIndicatorNode = JumpIndicator
 export declare class JumpIndicator {
   constructor(period: number, threshold: number)
@@ -1574,6 +1630,15 @@ export declare class KasePermissionStochastic {
   isReady(): boolean
   warmupPeriod(): number
 }
+export type VolatilityRatioNode = VolatilityRatio
+export declare class VolatilityRatio {
+  constructor(period: number)
+  update(high: number, low: number, close: number): number | null
+  batch(high: Array<number>, low: Array<number>, close: Array<number>): Array<number>
+  reset(): void
+  isReady(): boolean
+  warmupPeriod(): number
+}
 export type StochNode = Stochastic
 export declare class Stochastic {
   constructor(kPeriod: number, dPeriod: number)