perf(inline): unclosed [ and emphasis openers parse in linear time

bench/unclosed-brackets.test.cjs

@@ -0,0 +1,27 @@
+"use strict";
+
+// Unclosed `[`×N used to make findClosingBracket re-scan the remainder for every
+// opener (quadratic). After the per-parse memo, the first failed scan marks the
+// tail and the rest is O(1).
+const test = require("node:test");
+const assert = require("node:assert");
+const { loadMarkengineInRealm } = require("../tests/support/loadMarkengine.cjs");
+
+const E = loadMarkengineInRealm();
+const BUDGET_MS = 1000;
+
+test(`[×50000 parses inside ${BUDGET_MS} ms`, () => {
+  const source = "[".repeat(50000);
+  const started = performance.now();
+  E.parse(source);
+  const took = Math.round(performance.now() - started);
+  assert.ok(took < BUDGET_MS, `[×50000 took ${took} ms`);
+});
+
+test(`"*a "×10000 still inside 3000 ms (linear after no-close memo)`, () => {
+  const source = `${"*a ".repeat(10000)}x`;
+  const started = performance.now();
+  E.parse(source);
+  const took = Math.round(performance.now() - started);
+  assert.ok(took < 3000, `*a ×10000 took ${took} ms`);
+});