Add project documentation: design spec, implementation plan, and progress tracking

Initial project setup with README, CLAUDE.md (AI dev context), DESIGN.md
(full technical design covering architecture, data model, coordinate system,
rendering strategy), PROJECT_PLAN.md (phased implementation steps), and
PROGRESS.md (completion tracking).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -0,0 +1,27 @@
+eng-pad
+=======
+
+This is an Android app, targeting Android 11, and specifically the
+following two devices:
+
+1. The Supernote Manta (Android 11)
+2. The Daylight DC-1 (Android 13)
+
+There are a few features it needs to have:
+
+1. It is organized around notebooks.
+2. A *regular* notebook's page size is 8.5 x 11 inches. A *large*
+   notebook's size is 11 x 17 inches.
+3. The app features a grid of roughly 5 squares per inch. The grid is
+   not visible on exported documents.
+4. The app should allow pinch to zoom in and out.
+5. There should be two pen sizes, corresponding roughly to a Muji gel
+   ink ballpoint 0.38 or 0.5 pen.
+6. It should have the ability to erase, or to select-and-erase.
+7. It should have the ability to select and cut/move or copy/paste.
+8. The primary method of input is an EMR pen.
+9. The app needs to be able to export PDFs though the normal Android
+   sharing mechanism (e.g. so docs can be exported to Dropbox).
+
+The tech stack can be flexible. The safe default is Java, but if Kotlin
+or even Clojure or some other langauge makes more sense, it can be used.