refactor: add shared core foundations

[Foxpace]

Introduces the shared core abstractions used by the refactored phone and wearable applications.

What was added

• Typed application errors and diagnostic-report generation.
• Immutable preference state, intents, reducer, repository interface, and DataStore implementation.
• Native document storage for creating and writing measurement output.
• Unit tests, fixtures, and a fake preference repository for downstream modules.

How it works

Feature modules depend on the core interfaces instead of Android UI classes. Preference intents are reduced into state and persisted through DataStore, while storage requests are handled through the native document API. Errors are converted into structured diagnostics that can be shared by the UI.

Stack position

Layer 1 of 31 in the SensorBox refactor stack. Review this PR against its configured base to see only this layer.

Validation

• Full stack: ./gradlew test — BUILD SUCCESSFUL (160 tasks)

[Foxpace]

Superseded by #46, which targets master and shows the final current tree. The replacement PR body carries forward the review-thread accounting from this PR. Closing this stacked layer so review stays on the consolidated diff; this discussion remains available as history.

[Foxpace]

Stack restored at the author's request. This PR is again part of the original 31-layer refactor chain. The architecture work continues as stacked PRs #47 through #54; consolidated PR #46 is superseded.