# Proposal: Android Auto EQ preset selection

## Intent

Drivers cannot change the audio equalizer while on Android Auto — EQ selection is phone-UI only today. Expose the 6 fixed presets as a browsable folder in the existing Auto media tree so a preset can be applied from the car, without reaching for the phone and without interrupting the current station.

## Scope

### In Scope
- New top-level browsable `Ecualizador` folder in `ConstructorArbolAuto.raiz()`, listing the 6 fixed `PresetEcualizador.presets` by name.
- New media-id scheme `eq_preset:<nombre>`, intercepted in `playFromMediaId` BEFORE the `emisora:`/`grupo:` routing.
- Tapping a preset applies it through the existing headless-safe EQ seam and persists it as the principal preset (phone/car parity), with NO call to `playMediaItem` and NO `mediaItem` mutation.
- Strict-TDD tests: tree shape, `eq_preset:` id resolution, and the "preset tap does not disturb playback / now-playing" invariant.
- Optional (lower priority): mark the active preset in its row title (see Risks — no native selection UI exists in this model).

### Out of Scope
- Custom band-level EQ editing or arbitrary EQ from the car (media-browser capability ceiling; stays phone-only).
- Per-station / per-device / matrix EQ writes from the car (global principal only).
- Custom transport-button EQ actions (button real-estate fits ~1-2 slots, not 6 named presets).
- Favorite-groups browse work (separate, already-shipped `android-auto-favorite-groups`).

## Capabilities

### New Capabilities
- None

### Modified Capabilities
- `android-auto-media`: the browse tree gains an `Ecualizador` root folder plus an `eq_preset:` selection action that applies an EQ preset without starting or altering playback.

## Approach

Mirror the proven `emisora:`/`grupo:` interception pattern already in `navegacion_auto.dart` + `servicio_audio.dart`. Preset rows are `playable: true` (the only tappable non-folder item type in the legacy `MediaBrowserService` model) and therefore route through `playFromMediaId`; a new `eq_preset:` branch resolves the preset by name and calls the EQ apply seam instead of the audio player. Reuse the phone's global-preset semantics (apply via `PluriWaveAudioHandler.aplicarPreset` + persist via `ServicioEcualizador.guardarPrincipal`, matching `cambiarPresetPrincipal`) so phone and car stay in sync. The write path MUST be headless-safe (handler + service, NOT the widget-tree `EstadoEcualizador`, which may never build on an Auto bind).

## Affected Areas

| Area | Impact | Description |
|------|--------|-------------|
| `lib/servicios/navegacion_auto.dart` | Modified | Add `Ecualizador` root folder, `eq_preset:` id scheme, preset-row builder, resolver + selection routing helper |
| `lib/servicios/servicio_audio.dart` | Modified | Branch `playFromMediaId` on `eq_preset:` to the apply/persist seam before `emisora:`/`grupo:` |
| `lib/modelos/preset_ecualizador.dart` | Read-only | Source of the 6 fixed presets (name is the id key) |
| `lib/servicios/servicio_ecualizador.dart` | Read-only reuse | `guardarPrincipal` persistence seam (headless-safe) |
| `openspec/specs/android-auto-media/spec.md` | Delta only | Modified-capability delta in this change folder; base spec untouched |

## Risks

| Risk | Likelihood | Mitigation |
|------|------------|------------|
| Preset tap looks like "now playing a new track" (playable item) | Med | Never call `playMediaItem`, never mutate `mediaItem`; dedicated `eq_preset:` branch + invariant test |
| Shared-file merge drift with just-shipped favorite-groups | Med | Read CURRENT post-`f368bcc`/`066fedb` state; add branch alongside existing `grupo:`/`emisora:` dispatch |
| No native "selected" UI for browsable rows | High | Active-preset marker only via title text (prefix); ship as optional, be explicit it is not a real checkmark |
| Persist path unavailable on headless Auto bind | Med | Route through service + handler (SharedPreferences), not `EstadoEcualizador` |

## Rollback Plan

Additive only — revert the feature commits. No schema/migration changes: removing the `Ecualizador` folder and the `eq_preset:` branch restores the exact prior tree (favorite-groups behavior unaffected).

## Dependencies

- Sequenced after `android-auto-favorite-groups` (now on main) to avoid conflicts in the two shared files.

## Success Criteria

- [ ] `Ecualizador` folder appears at the Auto root with all 6 presets by name.
- [ ] Tapping a preset applies it (native EQ gains change) and persists as principal.
- [ ] Current station playback / now-playing metadata is unchanged by a preset tap.
- [ ] Selecting a preset in the car is reflected in the phone EQ UI and vice versa.
- [ ] Unknown/stale `eq_preset:` id is a no-op (no throw).