[EPIC] Feature-first structure — dissolve the single-library part-of monolith

The entire app is one Dart library: `lib/main.dart` declares **79 `part` directives** (main.dart:63-148) and every file opens with `part of carp_study_app`. There are no imports between app files, so nothing can express or enforce a boundary — any file can touch any other file's privates, which is the structural root of the UI/business-logic entanglement (#604).

Goal: proper libraries with explicit imports, organized by feature, migrated incrementally so the ~127 `bloc.*` call sites (counted in #597) move at most once.

**Definition of done:** no `part of carp_study_app` remains; `lib/features//` folders exist with a documented dependency rule (features → core, never feature → feature).

Line references are against branch `chore/cams-2.x-upgrade` @ 560a181.