Add P-touch Labels for Asset: 63x20 mm label format for PT-E560BT

New three-column label format for Brother P-touch EDGE PT-E560BT asset tags:
logo | three static lines + variable field | QR code.

Layout and rendering
- ptouch_label.py: geometry engine with PDF and preview renderers. Both
  consume the same geometry, so the preview matches the print exactly.
- Code symbols are snapped to whole printer dots per module. Fractional
  modules are the usual reason a small QR code will not scan.
- QR is maximised to the printable height by default (16.4 mm, 4 dots per
  module) instead of a fixed 16 mm box (12.3 mm, 3 dots per module).
- Code payload is composed as prefix + inventory + suffix. A legacy
  {inventory} template is migrated on first save.

Hardware handling
- print_area.py reads the driver's real printable area via GetDeviceCaps.
  The PT-E560BT head is 128 dots at 180 dpi, so the maximum print height is
  18.06 mm even on 24 mm tape; the driver reports media as 65x24 mm but only
  about 61x18 mm is reachable. Content is fitted to the reachable area,
  otherwise the driver silently crops it.
- set_printer_resolution.py reads/switches the driver's resolution mode. The
  driver offers 180x180 (symmetric) and 180x360 (anisotropic, 2x across the
  tape); the anisotropic mode scaled output incorrectly.

GUI
- label_printer_ptouch_gui.py: 600x300 two-column panel - controls on the
  left, live preview on the right. Advanced settings for logo, static lines,
  code prefix/suffix and error correction.
- native_file_dialog.py: the standard Windows file picker, run in a
  subprocess because Tk and Kivy both want the main event loop.

Printing
- Print failures are reported honestly. print_to_printer previously returned
  True unconditionally, so a failed print claimed success and was logged as
  SUCCESS.
- SumatraPDF exit codes are now checked (4 = no printer, 5 = driver error).
- The PDF is sent with noscale so driver-side scaling cannot distort the QR
  module pitch.

Tests
- validate_ptouch_layout.py decodes the QR back out of the rendered preview
  module by module, proving the preview and PDF are the same layout.
- test_ptouch_pipeline.py writes to a temp config so it cannot overwrite
  operator settings.
- test_gui_layout.py exercises the real app class headlessly.
- test_print_reporting.py covers the failure paths.

Diagnostics: check_print_area.py, check_print_geometry.py, check_devmode.py,
print_calibration.py, print_measure_scale.py, capture_print_job.py.

README_PTOUCH.md

@@ -0,0 +1,86 @@
+# P-touch Labels for Asset
+
+Barcode label printing for **Brother P-touch EDGE PT-E560BT** asset tags.
+
+Generates a three-column label sized to the printer's real printable area, with a
+logo, three lines of fixed text plus a variable field, and a QR code that encodes
+a composed string. Includes a preview that matches the print exactly, and a
+Kivy GUI for operators.
+
+```
+     0        20 mm        45 mm    63 mm
+     +---------+------------+--------+
+     |         | static 1   |  QR    |
+     |  LOGO   | static 2   |  CODE  |
+     |  16x16  | static 3   |        |
+     |         | VARIABLE   |        |
+     +---------+------------+--------+
+```
+
+## Install
+
+```bash
+pip install -r requirements_ptouch.txt
+```
+
+## Run
+
+```bash
+python label_printer_ptouch_gui.py      # operator GUI (Kivy)
+```
+
+The original three-row format is still available via `label_printer_gui.py` and
+is unchanged.
+
+## Configuration
+
+Operator settings live in `label_config.json` (not committed - it holds a local
+logo path). Edit them in **Advanced settings** in the GUI:
+
+| Setting | Purpose |
+|---|---|
+| Logo image | PNG/JPG for the first column, with a thumbnail preview |
+| Static text lines | The three fixed lines |
+| Code prefix / suffix | Code payload is `prefix + inventory + suffix` |
+| QR error correction | `L` (shortest) to `H` (most robust) |
+
+## Hardware notes
+
+The PT-E560BT has a **128-dot print head at 180 dpi**, so the maximum print height
+is **18.06 mm** even on 24 mm tape. The label is therefore fitted to the driver's
+*printable area* (~61 x 18 mm), read at runtime, rather than to the nominal media
+size (65 x 24 mm).
+
+The driver exposes two resolution modes; use the symmetric one:
+
+```bash
+python set_printer_resolution.py --mode 180
+```
+
+See [documentation/PTOUCH_63x20.md](documentation/PTOUCH_63x20.md) for the full
+layout, code-composition and dot-alignment details.
+
+## Diagnostics
+
+| Script | Purpose |
+|---|---|
+| `check_print_area.py` | Driver page and printable area (authoritative `GetDeviceCaps`) |
+| `check_print_geometry.py` | Compares our PDF page against the driver's media |
+| `check_devmode.py` | Dumps the full driver DEVMODE |
+| `set_printer_resolution.py` | Read/switch the driver resolution mode |
+| `print_calibration.py` | Label with ruler marks, for measuring on tape |
+| `print_measure_scale.py` | Measures the actual X/Y scale factors |
+| `capture_print_job.py` | Captures raw bytes sent to the printer |
+
+## Tests
+
+```bash
+python validate_ptouch_layout.py   # geometry + QR round-trip from rendered pixels
+python test_ptouch_pipeline.py     # config, PDF, print path (uses a temp config)
+python test_gui_layout.py          # GUI wiring, headless
+python test_print_reporting.py     # print failures are reported honestly
+```
+
+`validate_ptouch_layout.py` decodes the QR back out of the rendered preview,
+module by module, which is what proves the preview and the PDF are the same
+layout and that the symbol survives dot-alignment.