E16: add a cargo-fuzz harness

The repository had no fuzzing setup despite several natural targets —
the CSV parser, the Binance envelope deserializer, and the stateful
indicator/aggregator update paths.

Add a fuzz/ cargo-fuzz crate (detached from the workspace via its own
[workspace] table and the parent's exclude) with four targets:

- csv_reader      — CandleReader over arbitrary bytes
- binance_envelope — RawWsEnvelope deserialization from arbitrary strings
- indicator_update — RSI/EMA streaming + batch over arbitrary f64 series
- tick_aggregator — TickAggregator over arbitrary tick triples

Each target asserts the no-panic contract: malformed input must surface
as an Err. fuzz/README.md documents running them (nightly + cargo-fuzz).

fuzz/fuzz_targets/binance_envelope.rs

@@ -0,0 +1,13 @@
+#![no_main]
+//! Fuzz the Binance combined-stream envelope deserializer.
+//!
+//! `RawWsEnvelope` is what a kline frame is decoded into; feeding it
+//! arbitrary strings exercises the serde path that runs on every WebSocket
+//! frame. It must reject malformed input with an `Err`, never panic.
+
+use libfuzzer_sys::fuzz_target;
+use wickra_data::live::binance::RawWsEnvelope;
+
+fuzz_target!(|data: &str| {
+    let _ = serde_json::from_str::<RawWsEnvelope>(data);
+});

fuzz/fuzz_targets/csv_reader.rs

@@ -0,0 +1,17 @@
+#![no_main]
+//! Fuzz the OHLCV CSV reader with arbitrary byte input.
+//!
+//! The reader must never panic: malformed headers, non-numeric cells,
+//! truncated rows and arbitrary binary data all have to surface as an
+//! `Err`, never a crash.
+
+use libfuzzer_sys::fuzz_target;
+use wickra_data::csv::CandleReader;
+
+fuzz_target!(|data: &[u8]| {
+    if let Ok(mut reader) = CandleReader::from_reader(data) {
+        for candle in reader.candles() {
+            let _ = candle;
+        }
+    }
+});

fuzz/fuzz_targets/indicator_update.rs

@@ -0,0 +1,22 @@
+#![no_main]
+//! Fuzz indicator updates with arbitrary `f64` sequences.
+//!
+//! Every indicator must tolerate any finite-or-not input stream — NaN, ±inf,
+//! subnormals, abrupt jumps — without panicking, and `batch` must agree with
+//! the streaming `update` path.
+
+use libfuzzer_sys::fuzz_target;
+use wickra_core::{BatchExt, Ema, Indicator, Rsi};
+
+fuzz_target!(|data: Vec<f64>| {
+    let mut rsi = Rsi::new(14).unwrap();
+    let mut ema = Ema::new(20).unwrap();
+    for &x in &data {
+        let _ = rsi.update(x);
+        let _ = ema.update(x);
+    }
+
+    // batch over the same data must not panic either.
+    let _ = Rsi::new(14).unwrap().batch(&data);
+    let _ = Ema::new(20).unwrap().batch(&data);
+});

fuzz/fuzz_targets/tick_aggregator.rs

@@ -0,0 +1,25 @@
+#![no_main]
+//! Fuzz the tick-to-candle aggregator with arbitrary `(price, volume,
+//! timestamp)` triples.
+//!
+//! The aggregator must never panic — out-of-order ticks and volume overflow
+//! have to surface as an `Err`, and `Timeframe::floor` must not overflow for
+//! any `i64` timestamp.
+
+use libfuzzer_sys::fuzz_target;
+use wickra_core::Tick;
+use wickra_data::aggregator::{TickAggregator, Timeframe};
+
+fuzz_target!(|data: Vec<(f64, f64, i64)>| {
+    let mut agg = TickAggregator::new(Timeframe::new(60).unwrap()).with_gap_fill(true);
+    for (price, volume, ts) in data {
+        let Ok(tick) = Tick::new(price, volume, ts) else {
+            continue;
+        };
+        if agg.push(tick).is_err() {
+            // An out-of-order tick is a defined error; stop feeding this run.
+            break;
+        }
+    }
+    let _ = agg.flush();
+});