docs(structure): agree and document the feature map and dependency rule

## Problem
(verified) Current folders are by layer (`blocs/`, `data/`, `ui/`, `view_models/`); a page's model, view, and data access live in four directories, so boundary violations don't show up in review.

## Proposed fix
Short ADR in `docs/` defining: `core/` (sensing, backend, settings, resources — the #597 services), `features/onboarding`, `features/study` (home/messages), `features/tasks`, `features/data_viz`, `features/devices`, `features/profile`, and `shared/ui`. Rule: features import core/shared only.

## Acceptance criteria
- [ ] ADR merged with team sign-off
- [ ] Map covers every existing file

**Effort:** S · **Priority:** P1 · **Change risk:** None