feat(spec): add keyedBy(element, tags) identity helper

Joins element.find({testTag: tag})?.text per tag with U+001F as the
delimiter so user-visible text can never collide with the separator.
Returns empty string for an undefined element.

pkg/spec/src/extract.ts

@@ -1,5 +1,17 @@
-import type { Extracted, State } from "./types.ts";
+import type { AccessibilityElement, Extracted, State } from "./types.ts";
 
 export function extract<T>(getter: (state: State) => T): Extracted<T> {
   return globalThis.__sanderling__.extract(getter);
 }
+
+const KEY_DELIMITER = "\x1f";
+
+export function keyedBy(
+  element: AccessibilityElement | undefined,
+  tags: readonly string[],
+): string {
+  if (!element) return "";
+  return tags
+    .map(tag => element.find({ testTag: tag })?.text ?? "")
+    .join(KEY_DELIMITER);
+}

pkg/spec/src/index.ts

@@ -23,7 +23,7 @@ export type {
   WeightedEntry,
 } from "./types.ts";
 
-export { extract } from "./extract.ts";
+export { extract, keyedBy } from "./extract.ts";
 export { always, eventually, next, now } from "./ltl.ts";
 export {
   InputText,

pkg/spec/test/api.test.ts

@@ -12,6 +12,7 @@ import {
   eventually,
   extract,
   from,
+  keyedBy,
   next,
   now,
   pressKey,
@@ -281,6 +282,47 @@ test("from forwards items to the runtime", () => {
   assert.equal(sampler.generate(), "a");
 });
 
+function elementWithChildren(
+  cells: Record<string, string>,
+): AccessibilityElement {
+  return {
+    find: selector => {
+      if (typeof selector === "string" || Array.isArray(selector)) return undefined;
+      const tag = (selector as Record<string, string>).testTag;
+      if (!tag) return undefined;
+      const text = cells[tag];
+      if (text === undefined) return undefined;
+      return { text, find: () => undefined, findAll: () => [] };
+    },
+    findAll: () => [],
+  };
+}
+
+test("keyedBy joins testTag-resolved texts with a stable delimiter", () => {
+  installFakeRuntime();
+  const row = elementWithChildren({
+    TxnDate: "2026-04-26",
+    TxnNote: "Coffee",
+    TxnAmount: "$5.00",
+  });
+  const key = keyedBy(row, ["TxnDate", "TxnNote", "TxnAmount"]);
+  assert.equal(key, "2026-04-26\x1fCoffee\x1f$5.00");
+});
+
+test("keyedBy returns empty string for an undefined element", () => {
+  installFakeRuntime();
+  assert.equal(keyedBy(undefined, ["TxnDate"]), "");
+});
+
+test("keyedBy substitutes empty strings for missing children", () => {
+  installFakeRuntime();
+  const row = elementWithChildren({ TxnDate: "2026-04-26" });
+  assert.equal(
+    keyedBy(row, ["TxnDate", "TxnNote", "TxnAmount"]),
+    "2026-04-26\x1f\x1f",
+  );
+});
+
 test("default generators proxy through to the runtime", () => {
   installFakeRuntime();
   assert.equal(taps.__sanderlingActionGenerator, true);