Adds `make perf-baseline` — boots the full server plugin stack with real content, measures 50 ticks (5 warmup), captures per-tick timing, entity counts, and process RSS. Includes shadowcast benchmarks. Saves structured JSON to tests/perf/baseline.json for regression detection. Supports --compare mode (>20% threshold). First baseline: mean 366µs, p95 526µs (0.5% of D-026 100ms budget). Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>