test: add fast-check property suite

CLAUDE.md

@@ -94,6 +94,8 @@ Inline `for await` blocks **without** the IIFE wrapper will deadlock under fake
 
 For testing rejections, prefer the `.catch(err => ({ rejectedWith: err }))` envelope pattern (see `test/abort.spec.js`) over chai-as-promised's `should.be.rejectedWith` when asserting identity-equality on non-Error reasons; for exactly-once-delivery assertions across several pulls, use `collectNextOutcomes` + `expectSingleRejectionThenDone` from `test/utils.js` (the accumulator idiom used across `test/errors-fail-fast.spec.js`).
 
+`test/properties.spec.js` is the fast-check property suite — an accumulating background search, not a regression pin. Seeds stay random in CI (do NOT fix the seed — that converts accumulating coverage into a static test); a failure prints seed+path and replays deterministically via `FC_SEED`/`FC_PATH`, with `FC_NUM_RUNS` for deep local sweeps. **Anything a property finds must be pinned as a plain example spec in the relevant file** — regression protection never depends on run count. Property sources are timer-free (same rule as the benchmarks) and the file runs without sinon fake timers. The models (defensive source, request-queue semantics, abort anchoring) are the review surface: historically, property failures were model bugs more often than library bugs — verify against native `for await` before treating one as a finding. Hostile-shape testing (Proxies, throwing getters) deliberately stays in `test/hostile-results.spec.js` — `fc.anything()` doesn't generate those.
+
 `test/memory.spec.js` is the heap-retention regression guard for the per-pull park invariant. It runs under `--expose-gc` (wired via `.mocharc.json`'s `node-option: ["expose-gc"]`) and self-skips if `globalThis.gc` is unavailable (so `npx mocha test/memory.spec.js` directly still works — it picks up `.mocharc.json`). The other specs are unaffected by the flag.
 
 ## Benchmarks

package.json

@@ -77,6 +77,7 @@
     "chai-as-promised": "^7.1.2",
     "chai-quantifiers": "^1.0.18",
     "eslint": "^9.39.4",
+    "fast-check": "^4.8.0",
     "husky": "^9.1.6",
     "installed-check": "^9.3.0",
     "knip": "^5.30.2",

test/abort.spec.js

@@ -356,6 +356,10 @@ describe('bufferedAsyncMap() options.signal', () => {
     await after.should.eventually.deep.equal({ done: true, value: undefined });
   });
 
+  // This deterministic 11-offset sweep is the regression PIN for the
+  // drain-race identity contract; test/properties.spec.js's abort property
+  // is the accumulating SEARCH over the surrounding geometry (same
+  // commit-point oracle). Neither replaces the other — keep both.
   it('delivers exactly one rejection when an abort races the drain-throw, with the winner owning the identity', async () => {
     // The window: fail-eventually captured an error and the buffer drained;
     // an external abort landing in the await-gap between error capture and