test: expand complex workflow fuzz coverage

[senamakel]

Summary

• add generated cancellation, scatter isolation, timing-schedule, reducer associativity, and nested $replace properties
• add complex agent/sub-workflow/scatter/gather/loop fixtures with observer-order assertions, lane error policies, routed lane recovery, and a bounded 256-lane run
• add JSON reference workflows for spawn/gate and scatter/gather
• check in shrunk proptest counterexamples

Bugs fixed

• cancel spawned tickets, including tickets stored in lane slots, when a workflow is cancelled
• interpret nested $replace sentinels when a merge creates a new object subtree
• preserve lane-local state for on_error: continue and on_error: route
• hand stopped lane failures to gather.on_lane_error instead of aborting before the gather
• route lane activations only through the port actually emitted

Validation

• cargo test --all-features
• cargo clippy --all-features --all-targets -- -D warnings
• cargo fmt --all -- --check

Refs #46

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Review details

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Configuration used: Organization UI

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Run ID: 848f9d48-b9c9-402a-9eda-60a11654d5ea

📥 Commits

Reviewing files that changed from the base of the PR and between 2ca36c4 and d6adb1b.

📒 Files selected for processing (9)

• proptest-regressions/engine_merge_tests.txt
• src/engine.rs
• src/engine_merge_tests.rs
• tests/complex_graphs_tests.rs
• tests/fuzz_cancellation_tests.proptest-regressions
• tests/fuzz_cancellation_tests.rs
• tests/fuzz_scatter_tests.rs
• tests/fuzz_timing_tests.rs
• tests/reference_workflows_tests.rs

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