Scan the enrollment QR in either polarity

ai-server prints the QR as block characters, which take the terminal's
foreground colour. On a dark-themed terminal that comes out as a
photographic negative, and ZXing looks only for a dark code on a light
ground, so the in-app scanner silently never matched it -- while the
phone's own camera app, which tries both, did.

Fixing it in the scanner rather than by having the server force its
colours means however the code gets displayed stops mattering: a light
terminal, a dark one, a screenshot someone inverted, a printout.

MIXED_SCAN alternates normal and inverted frames, so the cost is half the
frame rate at each polarity. Checked against the library's own
MixedDecoder off-device: the plain decoder reads the normal image and
returns nothing for the inverted one, and the mixed decoder reads both
within two frames.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QQz6R4kBQcWSHBNZMgBnjL

AGENTS.md

@@ -152,6 +152,13 @@ Established 2026-08-25, and it decides more than it looks like:
   `"""\n-----BEGIN CERTIFICATE-----` costs Android's `CertificateFactory`
   its preamble sniff, so it tries DER instead and fails at runtime with
   `ASN.1 ... DECODE_ERROR` — nowhere near the code that produced it.
+- **ZXing only looks for a dark code on a light ground.** The enrollment
+  QR is block characters in the terminal's foreground colour, so a
+  dark-themed terminal renders it as a negative and the in-app scanner
+  silently never matches — while the phone's own camera app, which tries
+  both, does. The scanner asks for `Intents.Scan.MIXED_SCAN`, which
+  alternates normal and inverted frames; keep it that way rather than
+  making the server dictate the colours.
 - **AGP 9 refuses `Provider`s in the source-set API**: generated sources go
   through `androidComponents.onVariants { it.sources.java?.addGenerated
   SourceDirectory(task, Task::outputDir) }`, which also carries the task