perf: Add parallel processing infrastructure

[CybotTM]

Summary

Adds core utilities for parallel documentation processing using pcntl_fork.

New Classes

Class	Purpose
CpuDetector	Cross-platform CPU core detection (Linux, macOS, BSD)
ProcessManager	Forked process management with timeout and cleanup

Features

CpuDetector:

• Detects cores via /proc/cpuinfo (Linux)
• Falls back to nproc command (GNU/Linux)
• Falls back to sysctl -n hw.ncpu (macOS/BSD)
• Configurable max workers cap (default 8)

ProcessManager:

• Non-blocking wait with configurable timeout (default 300s)
• Automatic SIGKILL for stuck processes after timeout
• Secure temp file creation with 0600 permissions
• Signal handlers for cleanup on SIGTERM/SIGINT
• Child process temp file tracking to prevent cleanup conflicts

Usage Example

// Detect optimal worker count
$workers = CpuDetector::detectCores(maxWorkers: 8);

// Fork and track children
$childPids = [];
for ($i = 0; $i < $workers; $i++) {
    $pid = pcntl_fork();
    if ($pid === 0) {
        // Child: clear inherited temp tracking
        ProcessManager::clearTempFileTracking();
        // ... do work ...
        exit(0);
    }
    $childPids[$i] = $pid;
}

// Wait with timeout
$result = ProcessManager::waitForChildrenWithTimeout($childPids);
// $result['successes'] = worker IDs that completed
// $result['failures'] = worker ID => error message

Test Coverage

• CpuDetector: 4 tests covering core detection and max workers

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Note: This is a WIP PR. The utilities are complete but integration with a parallel renderer is planned for a follow-up PR.

Test Plan

• All existing tests pass
• New unit tests for CpuDetector (4 tests)
• Code style checks pass

[jaapio]

I'm sorry, I think this will add a lot of extra complexity to this project. I had this idea in mind for quit a while, and I do understand how this would drastically improve the speed of this project. But before we go this path I would love to learn more about how you think the new method would make sure we do not end up with race conditions. How do different processes share results etc.

This whole topic is very new to me, I'm open to learn new things but as I will be the one maintaining this, I need to have full understanding of this topic before we go this route.

[CybotTM]

Initially I thought I will provide the integration part with another PR, but does not make much sense, right?

Note: This is a WIP PR. The utilities are complete but integration with a parallel renderer is planned for a follow-up PR.

So I will add it here as additional commits.
And the bigger picture is still: https://cybottm.github.io/render-guides/ - render complete TYPO3 Core Changelog in under 1 minute.