feat(ci): add performance baseline tooling (#499)

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>

Makefile

@@ -6,7 +6,8 @@ GODOT := $(shell command -v godot4 2>/dev/null || command -v godot 2>/dev/null)
        pre-pr pre-pr-lint pre-pr-build pre-pr-test pre-pr-validate pre-pr-fixtures \
        pre-pr-server pre-pr-client pre-pr-content \
        fixtures-client golden-diff golden-update \
-       checklist-validate checklist-generate
+       checklist-validate checklist-generate \
+       perf-baseline
 
 # --- Configuration ---
 
@@ -44,6 +45,7 @@ help:
 	@echo "  make golden-update      Regenerate golden file and stage for commit"
 	@echo "  make checklist-validate Validate checklist YAML against schema"
 	@echo "  make checklist-generate Validate checklists + print condition summary"
+	@echo "  make perf-baseline      Run performance benchmarks and save baseline"
 	@echo ""
 	@echo "  make pre-pr             Run all pre-PR checks (lint, build, test, validate, fixtures)"
 	@echo "  make pre-pr-server      Server-scoped pre-PR (lint, build, test, fixtures)"
@@ -280,6 +282,9 @@ checklist-validate:
 checklist-generate:
 	@tooling/validate-checklist
 
+perf-baseline:
+	@tooling/perf-baseline
+
 content-ron:
 	cd tooling/content-converter && cargo build --release
 	tooling/content-converter/target/release/content-converter --input content --output content-ron --verbose