test: fuzz dispute_open boundary cases

contracts/predictify-hybrid/fuzz/targets/dispute_open.rs

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+//! Cargo-fuzz target for `dispute_open` boundary cases.
+//!
+//! The actual fuzz implementation lives in
+//! `src/tests/dispute_open_fuzz.rs` (a proptest-based test module)
+//! to keep access to internal crate APIs.
+//!
+//! ## Run the proptest fuzz target
+//!
+//! ```bash
+//! cargo test -p predictify-hybrid -- dispute_open_fuzz
+//! ```
+//!
+//! ## Cargo-fuzz setup (future work)
+//!
+//! To run via `cargo fuzz`:
+//!
+//! 1. Install cargo-fuzz: `cargo install cargo-fuzz`
+//! 2. Add a `fuzz/Cargo.toml` that depends on `libfuzzer-sys` and this crate
+//! 3. Populate this file with a `fuzz_target!` macro wrapping the logic
+//!    from the proptest tests
+//!
+//! ## Boundary conditions covered
+//!
+//! | Category | Conditions |
+//! |----------|-----------|
+//! | Stake | `MIN_DISPUTE_STAKE` (10_000_000), just below, just above, i128::MAX, zero, negative |
+//! | Market timing | Before `end_time`, after `end_time`, after dispute window closes |
+//! | Anti-grief | Stake below floor, at floor, above floor |
+//! | Duplicates | Same user disputing twice |
+//! | Stake caps | Per-market per-user cap exceeded |
+//! | Reason | `None`, empty `Some`, long string |
+//!
+//! ## Security
+//!
+//! All cases assume `mock_all_auths()` is active (auth is tested separately).
+//! No `unwrap()` is used in production paths.