🪲 BUG-#68: Fix busy-wait loop and enable multiprocessing on macOS via fork context

[FernandoCelmer]

Description

• dotflow/core/workflow.py: Replaced multiprocessing.Process/Queue with get_context("fork") on POSIX and get_context("spawn") on Windows; removed is_darwin() platform checks and macOS fallback warnings; replaced busy-wait polling loop with blocking queue.get(timeout=0.1) catching queue.Empty; simplified SequentialGroup.run() by removing the intermediate thread layer; added _processes list for liveness tracking; added else fallback to mark tasks as TypeStatus.FAILED when a worker process terminates without reporting a result; imported TaskError for structured error wrapping
• tests/core/test_workflow_deadlock.py: New test file — verifies that Parallel and SequentialGroup do not deadlock when a task fails before putting to the queue
• tests/core/test_workflow_platform.py: New test file — verifies correct multiprocessing context selection per platform (fork on POSIX, spawn on Windows)
• tests/core/test_workflow_parallel.py: Updated Queue type assertion to use fork context via get_context()
• tests/core/test_workflow_sequential_group.py: Updated Queue type assertion to use fork context via get_context()
• requirements.txt: Added requests dependency

Motivation and Context

The Parallel and SequentialGroup modes had two critical bugs:

1. Busy-wait CPU spike: the original while not queue.empty(): queue.get() loop consumed 100% CPU while waiting for task results.
2. macOS silent failures: Python 3.8+ defaulted to spawn on macOS, causing bound methods to fail silently in child processes — the original code worked around this with is_darwin() checks that degraded to sequential execution.

Additionally, if a worker process terminated without calling queue.put() (e.g., due to an unhandled exception not caught by Execution), the collection loop would deadlock indefinitely.

This PR fixes all three issues: forces fork context on POSIX (enabling macOS parallel mode), replaces busy-wait with a timeout-based queue.get(), adds process-liveness detection to break out of the collection loop safely, and marks unreported tasks as FAILED to ensure on_failure callbacks fire correctly.

Closes #68

Types of changes

• Bug fix (change that fixes an issue)
• New feature (change which adds functionality)
• Documentation

Checklist

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the CHANGELOG
• I have updated the documentation accordingly

[FernandoCelmer]

🔍 Code Review

Code issues found: 2

See inline comments below.

[FernandoCelmer]

🔍 Code Review

Code issues found: 3

#	Severity	Comment
1	[Suggestion]	Windows spawn — bound method pickling risk
2	[Blocking]	SequentialGroup: broad exception + silent task masking
3	[Blocking]	Parallel: broad exception + silent task masking