packaging: README (NASA style) + possibilistic perf baseline + CI benchmark job

README: purpose-first (possibility not boolean, caller owns evidence/time),
install, verified quick start, concepts (result shape, overlays, temporal
context), API table, development commands, design notes.

benchmark: scripts/benchmark.js on @tenere/benchmark-lib — eight contours
(direct/union/denied/meta/overlay/binary checks, snapshot build/restore),
committed .rigor-baseline.json, exit 1 on high-severity regressions.

CI: benchmark job compares on push (continue-on-error), re-saves baseline
and uploads it as an artifact on tags; publish now depends on benchmark
passing as well as test.

package.json

@@ -34,7 +34,7 @@
     "test:perf": "RUN_PERF_TESTS=1 node --test --test-force-exit tests/engine/core-performance-targets.test.js",
     "generate:parser": "node scripts/generate-parser.js",
     "build:ast": "npm run generate:parser",
-    "benchmark": "node benchmarks/core-performance-benchmark.js",
+    "benchmark": "node --expose-gc scripts/benchmark.js",
     "benchmark:core": "node benchmarks/core-performance-benchmark.js",
     "benchmark:batch": "node benchmarks/batch-size-analysis.js",
     "benchmark:chain": "node benchmarks/chain-rule-benchmark.js",
@@ -47,7 +47,8 @@
     "benchmark:snapshot": "node benchmarks/arbiter-snapshot-boot-bench.js",
     "benchmark:sharded": "node benchmarks/sharded-snapshot-build.js",
     "benchmark:memory": "node benchmarks/memory-breakdown.js",
-    "benchmark:multi-hop": "node benchmarks/multi-hop-rule-bench.js"
+    "benchmark:multi-hop": "node benchmarks/multi-hop-rule-bench.js",
+    "benchmark:save": "node --expose-gc scripts/benchmark.js --save"
   },
   "keywords": [
     "zanzibar",
@@ -64,6 +65,7 @@
   },
   "devDependencies": {
     "@rigor/core": "*",
+    "@tenere/benchmark-lib": "^2.0.1",
     "fast-check": "^4.5.3",
     "peggy": "^5.0.6"
   }