VALIDATION-PLANES-04: Make a section optional

What it demonstrates

optional(section(…)) lets a slot be absent without a finding while the rest of the order still holds.

Builds on: VALIDATION-PLANES-03: Constrain section order

How it’s done

A TypeScript program against the library API; inline comments show the resulting values and behavior.

import { optional } from "markdown-contract";

const c = contract({ body: sections({ order: "strict" }, [
  section("Summary"),
  optional(section("Notes")),  // may be absent
  section("Details"),
]) });
// "Summary" then "Details" (no "Notes") validates clean

Surfaces exercised

• optional() / OptionalSpec (src/core/grammar.ts)
• strict order with a skippable slot (src/core/structure.ts unwrap/slotsOf)