Story-2024: Markdown serialization core + plain preset (substrate keystone)

This is the KEYSTONE of the editing-substrate epic (EPIC-2023). It installs markdown as the serialization substrate under EditableSurface and introduces a preset seam so later stories can turn on real formatting without touching the primitive again. Ships exactly one preset — plain — which reproduces today's behaviour with zero data migration and no visible change.

What landed

• @tiptap/markdown added (v3.27.1, matching the installed TipTap). It is included in the plain preset's extension set, so editor.getMarkdown() and setContent(md, { contentType: "markdown" }) now exist on the surface — the substrate every later preset builds on.
• New EditableSurfacePreset type + resolvePreset() (editable-surface-preset.ts). A preset resolves to a { extensions, toContent, toValue } strategy. The component is now preset-driven; preset defaults to "plain", so every existing call site (the Brief sections) is unchanged.
• Dirty detection now uses getMarkdown(), not getText(), compared against a markdown baseline captured at load/commit. For plain this behaves identically (no formatting is available), but it is the foundation: once a richer preset lets a user bold a word, that formatting change trips the unsaved-changes icon.

Why plain loads & saves as literal text (not via markdown)

The obvious reading — "plain just runs stored strings through setContent(md, { contentType: "markdown" }) ↔ getMarkdown()" — corrupts real data. Probing the installed library against representative counsel prose showed:

So markdown-parsing the plain preset would break three acceptance criteria at once (literal < stays safe, zero migration, no visible change). The plain preset therefore keeps the literal text path — toContent builds a paragraph-per-line ProseMirror doc with no HTML/markdown parsing, and toValue is getText({ blockSeparator: "\n" }). A bare string round-trips as itself; a <, a leading #, and ** all render verbatim. The markdown machinery is installed for later presets, not applied to plain data. When Story-B adds light/full, it flips those two functions to the markdown path and adds a toolbar — resolvePreset is the only file that changes.

Files

• src/components/shared/editable-surface-preset.ts — new: the preset type, strategy, and resolvePreset().
• src/components/shared/editable-surface.tsx — preset-driven load/save; markdown-baseline dirty detection.
• src/components/shared/editable-surface-preset.test.ts — new: substrate bare-string identity + plain-preset verbatim/<-safe round-trip.
• src/components/shared/editable-surface.stories.tsx — new PlainPresetLiteralText story proving the unchanged case.

Deliberately deferred (not this story)

• The light / full presets and the formatting toolbar — Story-B.
• No pnpm codegen run; no wire-contract change.

Verification

pnpm lint (clean), pnpm build (green), and the full pnpm test suite (996 tests, incl. the new round-trip test) all pass. The PlainPresetLiteralText Storybook story demonstrates a value containing <, <Exhibit-7>, and a leading # saving byte-for-byte unchanged.