docs: add architecture reference

docs/architecture.md

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+# Architecture
+
+## Overview
+
+ScanSlicer is a Qt desktop app split into UI mixins, controller mixins, canvas subsystems, and image-processing/autotune engines.
+
+Top-level startup path:
+
+1. `scanslicer.entry:main`
+2. Backend/environment resolution and validation
+3. `scanslicer.main:run_app`
+4. `QtScanSlicer` window composition
+
+## Module Map
+
+- `scanslicer/entry.py`
+  - CLI parsing
+  - Wayland/X11 backend selection
+  - Qt platform environment configuration
+- `scanslicer/main.py`
+  - `QtScanSlicer` main window lifecycle
+  - Theme handling and app icon resolution
+  - App bootstrap (`run_app`)
+- `scanslicer/controller*.py`
+  - UI action handlers split by responsibility
+- `scanslicer/ui_builders*.py`
+  - Widget construction for control panel, preview panel, and shared UI helpers
+- `scanslicer/canvas*.py`
+  - Interactive image canvas for box display/editing
+- `scanslicer/autoslicer.py`
+  - Detection/export engine wrapper
+- `scanslicer/autoslicer_detection.py`
+  - Detection pipelines and contour-to-box extraction
+- `scanslicer/autoslicer_feedback.py`
+  - Auto-tune profiles, candidate scoring, and feedback persistence
+- `scanslicer/params.py`
+  - Parameter model, defaults, and user presets
+- `scanslicer/slices.py`
+  - Box geometry model and polygon IoU helper
+
+## UI Composition
+
+`QtScanSlicer` inherits:
+
+- `ScanSlicerUiBuilderMixin` (UI construction)
+- `ScanSlicerControllerMixin` (behavior)
+
+Controller mixin composition:
+
+- `ScanSlicerStatusMixin` (status/logging/busy state)
+- `ScanSlicerCanvasActionsMixin` (box operations and preview toggles)
+- `ScanSlicerDetectionMixin` (worker orchestration)
+- `ScanSlicerImageIOMixin` (directory/image loading and save/export)
+- `ScanSlicerParamsMixin` (parameter/default/preset/autotune interactions)
+
+## Detection Execution Flow
+
+1. User action triggers `update_preview()`.
+2. Detection starts in `DetectionTask` (`QRunnable`) on `QThreadPool` (single worker).
+3. Worker runs `Autoslicer.autodetect_slices(...)`.
+4. On completion:
+  - Cache normal preview image
+  - Cache filter preview image
+  - Update canvas image
+  - Merge/update boxes
+5. Toolbar status and box state are refreshed.
+
+Key properties:
+
+- One detection worker at a time.
+- Busy state disables controls during processing.
+- Task IDs prevent stale callbacks from old tasks mutating UI.
+
+## Auto-Tune Execution Flow
+
+1. User clicks `Auto-tune`.
+2. `AutotuneTask` runs in the same single-thread pool.
+3. `autotune_for_current_image()`:
+  - Builds candidate parameter sets
+  - Ranks by current-image + prior feedback similarity
+  - Coarse pass on downscaled image (`fast_mode=True`)
+  - Fine pass on top finalists
+  - Applies best candidate to parameter model
+4. UI controls sync from model and detection re-runs.
+
+## Canvas System
+
+- `QtSlicingCanvas` = interaction + rendering + state mixins
+- Stores:
+  - image and zoom/pan state
+  - list of `PhotoSlice` objects
+  - active slice index
+  - drag modes (corner, side, rotation, box drag, panning)
+- Emits `active_slice_changed` signal to keep toolbar state synchronized.
+
+## Geometry Model
+
+`PhotoSlice` stores 4-point quadrilateral and flags:
+
+- `locked`
+- `side_handles_enabled`
+
+It supports:
+
+- top-left normalization and winding correction
+- rectangle detection (`is_perfect_rectangle`)
+- squarize while preserving tilt
+- reset to axis-aligned orientation
+
+## Persistence Model
+
+Paths are runtime-dependent (host vs container), with environment overrides:
+
+- Defaults JSON
+- Presets JSON
+- Auto-tune feedback JSON
+- Session status log
+
+Persistence writes are performed through explicit actions (save defaults/preset, save slices with feedback, status updates).
+
+## Export Pipeline
+
+For each locked box:
+
+1. Compute output width/height from opposing edge lengths.
+2. Build perspective transform from box quad to rectangle.
+3. Warp source image with `cv2.warpPerspective`.
+4. Save with format-specific encoding params.
+
+## Packaging Surfaces
+
+- Python package/CLI (`pyproject.toml`, script `scanslicer`)
+- Docker runtime (`Dockerfile`, `docker-compose.yml`)
+- AppImage (`packaging/appimage/`)