feat: add single-threaded baseline example for pthread comparison

Introduced a simple summation program without thread creation or synchronization.
Serves as a deterministic baseline to compare against multithreaded versions
that exhibit race conditions or mutex-based synchronization.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section19MultiThreading/Threading Inside Look/SingleThreadDemo.c

@@ -0,0 +1,55 @@
+#include <stdio.h>
+#include <pthread.h>
+#include <unistd.h>
+
+// Shared global variable accessed by all threads
+volatile long int a = 0;
+
+// Thread function 1 — performs addition from 1 to 499,999
+void* threadFunction1(void* args)
+{
+    int i;
+    long int localA = 0;
+    for (int i = 1; i < 500000; i++)
+    {
+        a = a + i;  // Update shared variable (not thread-safe)
+    }
+    return NULL;
+}
+
+// Thread function 2 — performs addition from 1 to 1,000,000
+void* threadFunction2(void* args)
+{
+    int i;
+    long int localA = 0;
+    for (i = 1; i <= 1000000; i++) {
+        a = a + i;  // Update shared variable (not thread-safe)
+    }
+    return NULL;
+}
+
+int main(int argc, char** argv)
+{
+    int i;
+    pthread_t one, two;
+    a = 0;  // Initialize shared variable
+
+    // Simple loop performing addition without threads
+    for(i = 1; i <= 500000; i++) {
+        a = a + i;
+    }
+
+    // Print final value (only single-threaded computation here)
+    printf("Final value of a = %ld\n", a);
+
+    return 0;
+}
+
+/*
+•	Runs only a single-threaded loop in main() — threadFunction1 and threadFunction2 are defined but never used.
+•	The final printed value of a comes only from the loop in main().
+•	Essentially, it’s not concurrent; it’s just a regular summation program.
+•	No thread creation (pthread_create) or joining (pthread_join) happens.
+
+Result: predictable and consistent, because there’s no threading or race condition.
+*/