Rebuild opencv2.xcframework against a release Xcode 26.x SDK

[bmc08gt]

#635 unblocked releases by restamping the committed opencv2.xcframework from
sdk 27.0 down to 26.1 with vtool. That fixes the LC_BUILD_VERSION load
command Apple validates (ITMS-90512), but it does not change the toolchain the
framework was actually compiled with — the binary is still OpenCV 4.12.0 built
by an Xcode 27.0 beta clang against the beta SDK headers.

That is fine to ship (the code is C++ with no beta-SDK API surface, and it is
the same source revision Android runs), but it should not stay that way.

To do

1. Install a release Xcode 26.x and make it active (sudo xcode-select -s).
2. Re-run CodeScanner/Scripts/build_opencv.sh — the new check_sdk_version
   guard from fix(scanner): stamp opencv2 with a released SDK #635 will confirm the SDK is acceptable rather than silently
   producing another unshippable framework.
3. Commit the regenerated opencv2.xcframework and verify with
   xcrun vtool -show-build-version that every slice reads sdk ≤ 26.5.
4. Smoke-test the scanner (Cash Link + code detection) before merging — the
   4.12.0 bump in fix(scanner): bring the detector back in line with Android, on matching OpenCV #630 came with a scanner.cpp change, so this is not a
   no-op rebuild.

Context

• Rejected build: 2026.8.3 build 500, INVALID_BINARY.
• Last known-good stamp: sdk 26.1, from the framework shipped in 2026.8.2.
• Only Xcode on the machine that produced the 4.12.0 framework: 27.0 beta
  (27A5228h).