Scaffold Android project per BRIEF.md

Single-module Kotlin/Compose app with manual DI container. Pure-Kotlin
measurement math (orientation mapping, two-sample per-mode calibration,
EMA smoothing, lock hysteresis, display deadband) fully unit-tested.
Sensor fallback: game rotation vector -> gravity -> low-passed
accelerometer. Live Level (manual Surface|Edge) and Angle (hold-to-zero)
scaffolds; Ruler/Tools placeholders; stub Pro entitlement.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -0,0 +1,42 @@
+# On the Level
+
+Offline-first Android pocket toolset: a trustworthy level, angle meter, and
+calibrated screen ruler (Pro). Kotlin + Jetpack Compose, single module, no backend.
+
+- **[BRIEF.md](BRIEF.md)** — the authoritative v1 product & technical brief
+- **[AUDIT.md](AUDIT.md)** — audit of the brief; the amendments it lists are incorporated
+- **resources/** — concept board (visual direction)
+
+## Building
+
+Open in Android Studio, or:
+
+```sh
+./gradlew assembleDebug        # build
+./gradlew testDebugUnitTest    # measurement-math unit tests
+```
+
+## Scaffold status
+
+The project skeleton follows BRIEF.md §Sensor and measurement architecture.
+Implemented and tested:
+
+- `core/sensors` — sensor selection (game rotation vector → gravity → low-passed
+  accelerometer), device-frame gravity stream, and the pure measurement math:
+  orientation mapping, two-sample per-mode calibration, EMA smoothing,
+  lock hysteresis/dwell/debounce, display deadband. All unit-tested.
+- `core/settings` — DataStore preferences and per-mode calibration persistence.
+- `core/billing` — entitlement interface with a stub (free-tier) implementation.
+- `feature/level`, `feature/angle` — live numeric scaffolds wired to the real
+  sensor pipeline, with manual Surface|Edge selection, lock state, hold-to-zero.
+- `feature/ruler`, `feature/tools` — static placeholders.
+
+Not yet implemented (deliberately — see TODO markers):
+
+- The tactile vial visuals and the spring-animated **display value** (value 3 of 3
+  in the brief); scaffold screens render the deadbanded stable value directly.
+- Guided calibration flow UI (math and persistence are done).
+- Edge-mode readout counter-rotation on the portrait-locked activity.
+- Play Billing implementation of `ProEntitlementRepository`.
+- Screen ruler measurement UI and per-display scale persistence.
+- Audio cue, reduced-motion behaviors, preferences UI.