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/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();
+});