Add memory-pressure benchmark for petclinic throughput testing

Adds a new benchmark category under benchmark/memory/petclinic/ that measures
tracer throughput overhead across decreasing heap sizes using spring-petclinic
3.3.0 and JMeter. Based on Doug's methodology for evaluating memory overhead
of the Java tracer agent.

The script auto-downloads JMeter and builds petclinic, iterates over
configurable heap sizes (default: 256/192/128/96/80/64 MiB), runs each with
and without the agent, and outputs a comparison table with throughput delta.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bm1549

benchmark/memory/petclinic/.gitignore

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+tools/
+results/

benchmark/memory/petclinic/README.md

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+# Memory Pressure Benchmark
+
+Measures tracer throughput overhead across decreasing heap sizes using spring-petclinic and JMeter.
+
+Based on Doug's methodology: run petclinic with `GET /owners/3` (DB-backed endpoint) at heap sizes from 256m down to 64m, comparing no-agent baseline vs candidate agent.
+
+## Quick start
+
+```bash
+# Full run: baseline + candidate at all heap sizes
+./run.sh
+
+# With a specific agent jar
+./run.sh --agent /path/to/dd-java-agent.jar
+
+# Quick test with fewer heap sizes and shorter durations
+./run.sh --heap-sizes 256,128,64 --warmup 15 --measure 30
+
+# With JFR profiling
+./run.sh --jfr
+
+# Only run candidate (skip baseline if you already have baseline numbers)
+./run.sh --skip-baseline
+
+# Test a specific optimization with extra agent opts
+./run.sh --agent-opts "-Ddd.appsec.enabled=false -Ddd.profiling.enabled=false"
+```
+
+## What it does
+
+For each heap size (default: 256, 192, 128, 96, 80, 64 MiB):
+
+1. Starts petclinic with `-Xms<N>m -Xmx<N>m` (no agent)
+2. Runs JMeter with 8 threads for warmup + measurement
+3. Records stabilized throughput (measurement window only)
+4. Repeats with `-javaagent:dd-java-agent.jar`
+5. If petclinic OOMs or fails to start, records "OOM"
+
+## Output
+
+```
+Heap      Baseline        Candidate       Delta
+------    -----------     -----------     -----------
+256m      22400 req/s     19500 req/s     -12.9%
+192m      21750 req/s     19000 req/s     -12.6%
+128m      21500 req/s     14750 req/s     -31.4%
+96m       16750 req/s     13000 req/s     -22.4%
+80m       16350 req/s     11250 req/s     -31.2%
+64m       15000 req/s     7500 req/s      -50.0%
+```
+
+Results are also saved as CSV in `results/<timestamp>/summary.csv` with per-run JTL files and logs.
+
+## Dependencies
+
+Auto-downloaded on first run:
+- **JMeter 5.6.3** → `tools/apache-jmeter-5.6.3/`
+- **spring-petclinic 3.3.0** → `tools/spring-petclinic/` (built with Maven)
+- **dd-java-agent** → auto-detected from `dd-java-agent/build/libs/` or built via Gradle
+
+Requires: Java 17+, Python 3 (for JTL parsing), curl, git.
+
+## Comparing versions
+
+To compare two tracer versions, run separately and diff the CSVs:
+
+```bash
+# Run with release version
+./run.sh --agent /path/to/dd-java-agent-1.61.0.jar --output ./results/v1.61.0
+
+# Run with candidate
+./run.sh --agent /path/to/dd-java-agent-SNAPSHOT.jar --output ./results/candidate
+
+# Compare
+paste -d, results/v1.61.0/summary.csv results/candidate/summary.csv
+```