practice: the learner's creation note — optional free-form input carried to partitioner + generators (owner, 2026-09-29)

- dialog: optional 'Anything else?' textarea (typing-only, 2000-char mirror)
- API: body gains note? — trimmed, whitespace-only omitted, hard 400 over 2000
- pipeline: note persisted on the set and threaded to both agent stages
- agents: THE LEARNER'S NOTE prompt block (present only when set) + the
  weighed-never-owed stance pinned in both doctrines
- fix en route: doubled THE PARAMETERS heading in both prompts since slice 3
  (renderParams owns the heading; the literal blocks removed)

tests/practice-store.test.ts

@@ -152,6 +152,13 @@ describe("isProblemSet", () => {
 		assert.ok(!isProblemSet({ ...init(), status: "cooked" }));
 	});
 
+	it("the learner's note: optional string, round-trips; non-string refused (owner, 2026-09-29)", () => {
+		assert.ok(isProblemSet({ ...init({ note: "keep it geometric" }), id: "ps_noted" }));
+		assert.ok(!isProblemSet({ ...init({ note: 42 as unknown as string }), id: "ps_badnote" }));
+		const created = practiceStore.createSet(init({ note: "lean toward pricing" }));
+		assert.equal(practiceStore.getSet(created.id)!.note, "lean toward pricing");
+	});
+
 	it("tolerates optional fields — a minimal stub (no error/answer/grade) reads", () => {
 		const minimal = { ...init({ problems: [{ id: "pp_1", chains: [], statement: "", solution: "", rubric: "", status: "generating" }] }), id: "ps_valid" };
 		assert.ok(isProblemSet(minimal));