Misc · Codes and checks: a QR code scanner and a captcha

misc::QrCodeScanner reads QR codes with rqrr 0.11 from the frames the
host hands in, one at a time off the main thread, and hands each code
to the owner once. misc::Captcha shows the owner's picture over an
answer field that empties and keeps focus with each answer; the owner
says how it stood. BarcodeDisplay, Signature and Rating Widget point
to their homes.

tasks/ch31-43.md

@@ -320,11 +320,11 @@
 
 ## T42c Misc · Codes and checks
 
-- [ ] QRCodeScanner — prove: host frames, `rqrr` decode
-- [ ] BarcodeDisplay → `data_display::Barcode`
-- [ ] Captcha
-- [ ] Signature → `forms::SignaturePad`
-- [ ] Rating Widget → `forms::Rating`
+- [x] QRCodeScanner — proven: `rqrr` 0.11 reads the host's frames off the main thread → `misc::QrCodeScanner`, a camera's view with a square to aim at; each code goes to the owner once
+- [x] BarcodeDisplay → `data_display::Barcode`
+- [x] Captcha → `misc::Captcha`: the owner's picture and verdict over an answer field that empties and keeps focus with each answer, with a refresh
+- [x] Signature → `forms::SignaturePad`
+- [x] Rating Widget → `forms::Rating`
 
 ## T42d Misc · Asking and learning