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>

PROGRESS.md

@@ -0,0 +1,30 @@
+# PROGRESS.md — eng-pad Implementation Progress
+
+This file tracks completed work and decisions. Updated after every step.
+See PROJECT_PLAN.md for the full step list.
+
+## Completed
+
+### Phase 0: Project Documents (2026-03-24)
+
+- [x] 0.1: Created DESIGN.md — full technical design covering architecture,
+  data model, coordinate system, rendering, input handling, and source tree.
+- [x] 0.2: Created PROJECT_PLAN.md — discrete steps grouped by phase with
+  checkboxes and file references.
+- [x] 0.3: Created PROGRESS.md — this file.
+- [x] 0.4: Updated CLAUDE.md — build commands, source tree, project doc
+  pointers.
+
+## In Progress
+
+Phase 1: Project Skeleton + Data Layer
+
+## Decisions & Deviations
+
+- **Language**: Kotlin (chosen over Java for better Compose/coroutine support).
+- **Storage**: Room/SQLite with packed float BLOBs for stroke points.
+- **Palm rejection**: Hardware only (EMR digitizer handles it).
+- **Pressure sensitivity**: None — two fixed pen sizes (0.38mm, 0.5mm).
+- **Coordinate system**: 300 DPI canonical points (scaled to 72 DPI for PDF export).
+- **UI framework**: Compose for chrome, custom View for canvas (Compose
+  Canvas lacks MotionEvent access needed for stylus input).