feat: add imageMode configuration option for raster and column image …

[GomdimApps]

Background

A user printing to a real MP58C7 (58mm clone thermal printer, connected
over Bluetooth) got images back as heavy banded/sheared garbage, while
text, barcode and QR printed perfectly fine on the exact same
connection. The on-screen preview looked correct — only the physical
print was broken.

Investigation ruled out, in order:

1. Image wire format — the underlying @point-of-sale/receipt-printer-encoder
   defaults to the legacy ESC * "column" band format unless a
   recognized printerModel is set, and only then switches to the
   single-shot GS v 0 "raster" format. This project never exposed
   that knob, so it was always silently on column for unrecognized
   printers. Turned out not to be the cause here: forcing raster
   produced byte-for-byte identical corruption on this hardware.
2. Bluetooth write chunking/pacing — tried the default (100
   bytes/write, no delay), smaller chunks (20, 8 bytes), and added
   inter-write delays (10-20ms), independently and combined. None of it
   changed the outcome; some combinations made it visibly worse.
3. Payload size — even a small synthetic ~256-byte test image
   corrupted the same way as a full-size photo.
4. The printer's firmware itself — turned out to be fine: the
   exact same encoded bytes (either wire format) printed perfectly
   when sent over Serial/USB instead of Bluetooth.

Conclusion: this printer family (MP58C7, also sold as MTP-II / HPRT
HM-A200U / PixPos MP58C7 — same underlying hardware) has a Bluetooth
transport-layer issue with image-sized payloads, most likely in its
BLE-to-serial bridge chip, that no amount of chunk-size/pacing tuning
from the browser side could work around. Since the library can't fix
that, this PR documents it as a known hardware limitation (with the
confirmed workaround) and adds the imageMode knob anyway, since it's
a legitimate config option other clone printers may need even though
it wasn't the fix here.

Summary

• Add an opt-in imageMode: 'column' | 'raster' config option (constructor and per-job), passed straight through to ReceiptPrinterEncoder — lets consumers force the single-shot GS v 0 raster wire format instead of the encoder's default legacy ESC * column/band format, for clone printers whose firmware handles one better than the other.
• Add two demo-only diagnostic tools (demo/index.html + demo/app.js): an "Image mode" selector, and "Print diagnostic pattern" / "Print diagnostic pattern (tiny)" buttons that print a known vertical-stripe test pattern at two payload sizes — used to pin down exactly how a printer garbles image output, independent of any real source image.
• Document the investigation above as a new gotcha (docs/notes/13, referenced from AGENTS.md and README.md): on the MTP-II/MP58C7 clone family, image printing over Bluetooth is unreliable regardless of imageMode, BLE write chunk size, or inter-write pacing. The same encoded bytes print correctly over Serial/USB, so that's the recommended transport for image content on this hardware family.
• Add a regression test (test/Images/image.test.ts) asserting the actual emitted command bytes differ between the two imageMode values (ESC 3 36/ESC * 33 for column vs. GS v 0 for raster), closing a gap where no test previously checked image command bytes at all.