added chunked Pi calculations test for fork vs thread

pi_digits_benchmark/README.md

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+# Pi Digits Benchmark
+
+This benchmark compares pthread-based parallelism and fork()-based parallelism
+for computing digits of pi in independent chunks.
+
+## Configuration (environment variables)
+
+- `THREAD_COUNT`  : number of worker threads
+- `PROCESS_COUNT` : number of worker processes
+- `PI_DIGITS`     : total digits of pi to compute
+- `CHUNK_SIZE`    : digits per worker
+
+Defaults: `THREAD_COUNT=4`, `PROCESS_COUNT=4`, `PI_DIGITS=2000`, `CHUNK_SIZE=500`
+
+## Run
+
+```sh
+# Run both modes sequentially
+bash run_pi_digits_benchmark.sh
+
+# Run a single mode
+bash run_pi_digits_benchmark.sh thread
+bash run_pi_digits_benchmark.sh process
+```
+
+## Outputs
+
+Per-run outputs are stored under `runs/`:
+
+- `pi_digits.txt` — final concatenated digits
+- `workers.csv` — per-worker durations and chunk metadata
+- `results.csv` — per-run summary metrics (includes aggregated RSS/VSZ and snapshot overhead)
+- `stdout.log` / `stderr.log` — benchmark logs