Draft PPTX import spec

Adds docs/PPTX_IMPORT_FEATURE.md as the canonical plan for the next
big arc: a phased editable .pptx importer that lands PowerPoint decks
as editable Artstr deck JSON.

Key decisions baked in:

- Images and unsupported objects (charts / SmartArt / tables / video)
  import as placeholder image layers pointing at a hardcoded URL
  (postimg.cc for now). Original PowerPoint media filenames carry
  over into the layer name so the user can correlate placeholders
  with their source deck.
- fflate (~10 KB) vendored as src/vendor/fflate.min.js. JSZip is too
  heavy for the single-file ethos.
- Importer code lives in src/pptx-importer.js as an IIFE that
  exposes window.ArtstrPptxImporter. The Import PPTX button
  dynamically injects both scripts on first click, so the main
  index.html bundle does not grow for the 90 % of users who never
  import a PPTX.
- Notes folded into Phase 2 alongside text (single extra relationship
  lookup, Presenter Mode already renders slide.notes).
- No Blossom / NIP-96 prereq, no base64 bloat, no publish-size guard.
  Imported decks are normal Artstr deck payloads with remote URLs.
- Import-only for this arc. PPTX export (Artstr -> .pptx) is a
  separate later arc with its own complexity.

TODO.md gets a Drafted-but-unbuilt entry pointing at the new spec.

TODO.md

@@ -66,6 +66,14 @@ toggle that fits the early-web aesthetic. Add new editor-UX items below.
 
 Each has a full spec in `docs/`.
 
+- **PPTX import** — `docs/PPTX_IMPORT_FEATURE.md`. Phased editable
+  importer that converts a `.pptx` package into an Artstr deck.
+  Images and unsupported objects (charts, SmartArt, tables, video)
+  land as placeholder image layers pointing at a hardcoded URL;
+  user replaces each placeholder by editing the layer's src. No
+  Blossom prereq, no base64 bloat, decks publish to Nostr normally.
+  fflate vendored bundle + a lazy-loaded `src/pptx-importer.js`
+  module so the main app weight doesn't grow.
 - **Artstr Stacks** — `docs/STACKS_FEATURE.md`. Interactive fullscreen
   page/card stacks (HyperCard for Nostr) — the Slide Deck extended with
   layer actions and an interactive viewer.