std.concurrency: Add join() to prevent thread resource leaks

[CyberShadow]

Add a new join(Tid) function that allows explicit joining of threads created by spawn(). This prevents OS resource leaks in long-running applications that create many short-lived threads.

Without calling join(), thread stacks (~8 MB each on typical systems) accumulate in virtual memory for the lifetime of the process because:

• Threads created by spawn() are never explicitly joined
• Thread objects remain in the global thread list until process exit
• pthread cannot free thread stacks until pthread_join() is called

The new join() function:

• Blocks until the thread completes
• Releases OS resources (stack, TLS) via pthread_join()
• Throws ThreadError if used on scheduler-created threads
• Throws ThreadError if the thread was already joined

Motivating example:

import std;
import core.thread;

void worker() { Thread.sleep(1.msecs); }

void printVmSize() {
    "/proc/self/status".readText.splitLines
        .filter!(l => l.startsWith("VmSize:"))
        .front.writeln;
}

void main()
{
    printVmSize();

    foreach (i; 0 .. 1000)
    {
        auto tid = spawn(&worker);
        version (doNotLeakMemory) tid.join();
    }

    Thread.sleep(100.msecs); // ensure all tasks finished

    printVmSize();
}

Output:

$ dmd std/**/*.d example_join.d -of=./t_join && ./t_join              
VmSize:	    9972 kB
VmSize:	33310900 kB

$ dmd std/**/*.d example_join.d -of=./t_join -version=doNotLeakMemory && ./t_join               
VmSize:	    9972 kB
VmSize:	   87948 kB

[dlang-bot]

Thanks for your pull request, @CyberShadow!

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10894"