build: domain modules and one app module per surface #286

Parent: #52

Split the single flavored :app into domain library modules and one application module per surface, so CI stops doing the same work once per variant and each domain's public interface is enforced by the compiler.

Decision (Altay): domain modules + two apps. Full feature × surface slices (~30 modules) and a package-only reorg were rejected: the first is over-modularized at ~35k lines, the second gives no build gain.

Why

• Shared src/main compiles once per variant: six times in verify, about 12s each locally.
• Shared unit tests and lint run once per surface, and lintVital runs once per release build.
• testEvidence and testEmulatorHarness declare no inputs, so they rerun on every build.
• Baseline at 3204b82, ./gradlew verify on an 18-core Mac after a one-line edit: TV-only 46s, mobile-only 53s, shared 76s. CI takes 9–21 min for code PRs.
• Google's modularization patterns say: "If your app targets multiple device types … define an app module for each one."

Target

core/common   core/design
domain/{files,playback,account,transfers,downloads,history,search,trash,auth}
mobile/   :mobile app, channel flavors, mobile UI in domain packages
tv/       :tv app, channel flavors, TV UI in domain packages

• Application ids, the channel flavors, Kotlin packages and the MainActivity FQN stay as they are.
• Domain types used by both apps become public; everything else stays internal.

Slices

• Cache the script-backed verification tasks: build: skip unchanged script checks in verify #287
• Extract core/* and domain/* from app/src/main, with their tests and test fixtures; :app stays flavored: build: move shared code into core and domain library modules #289
• Stop running the lintVital analysis that verify discards (about 126 task-seconds per build; found by the audit): build: stop running lintVital analysis that verify discards #290
• Replace :app and its surface flavor with the :mobile and :tv application modules; update the harness, CI and docs; turn detekt on for app code, with a baseline: build: split the app into mobile and tv application modules #291 (baseline burn-down: build: burn down the mobile and TV detekt baselines #292)

The interface tightening the audit turned up (state constructors, shared kernel, downloads placement) is tracked in #288.

Acceptance

• ./gradlew verify and every debug assemble are green after each slice. The surface split keeps all application ids identical (aapt check).
• scripts/prove.sh mobile and scripts/prove.sh tv pass on the final layout.
• Each PR records the before/after incremental verify benchmark.
• AGENTS.md, README and docs/harness.md describe the new layout and task names.

Written by an agent (Claude Code, Opus 5.5)