FLOGO-18450: Concurrent execution of parallel transition branches

[awakchau-tibco]

What

Adds opt-in concurrent execution of activities on parallel transition branches (fan-out) in the flow action. Branches modeled as parallel now run at the same time instead of one activity being processed at a time.

Why

The flow drains work sequentially, so independent branches were serialized — total latency equaled the sum of branch times. Concurrent execution reduces this to roughly the slowest branch, especially impactful for I/O-bound branches (delay / REST / DB).

Design & safety

• Gated behind the existing FLOGO_FLOW_CONCURRENT_TASK_EXECUTION env flag (default off). When off, the sequential DoStep loop is byte-for-byte unchanged, so no existing flow regresses unless explicitly opted in.
• When on, FlowAction.Run drives a new IndependentInstance.RunConcurrent worker pool (min(GOMAXPROCS, 32) workers) that runs ready tasks concurrently while preserving join semantics (a task runs only after all its incoming links resolve) and order-independent results.
• support/env.go: GetConcurrentExecution(); instance/util.go IsConcurrentTaskExcutionEnabled delegates to it (one source of truth, shared with the change-tracker lock).
• Two-lock design on IndependentInstance: coarse stateLock (traversal/scheduling, taskInsts/linkInsts/subflows) + attrsLock RWMutex (shared scope + returnData), both nil in sequential mode. Lock order: stateLock -> changeTracker -> attrsLock.
• stepID / wiCounter / subflowCtr promoted to atomics.
• Per-task cancellable context (TaskInst.evalCtx) so context-aware activities abort on sibling failure.
• Drain-then-fail: the first unhandled branch error cancels siblings, the pool waits for in-flight branches to return, then the global error handler fires once.
• execTask and the new execTaskConcurrent share extracted evalTaskBehavior + handleEvalResult helpers (removes duplication).

Testing

• Unit + integration tests (parallelism, join-fires-once, drain-then-fail, sequential parity), ~87% coverage of the new code; clean under go test -race.
• Fixed pre-existing map-order-flaky TestGetErrorObject_* (def.Tasks()[0] -> def.GetTask("LogStart")).

Performance

• Benchmarks + a when-to-enable guide in instance/PARALLEL_BENCHMARKS.md. Crossover ≈ 50–100 µs of work per branch; recommend enabling only for 2+ branches each doing blocking I/O or ≳ ~100 µs of CPU.

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