Add native Windows Arm64 library readiness - #1089

Automated review is being run on mirror PR yeelam-gordon#1. Findings and fixes are pushed to this same head branch. This PR depends on OpenShot/libopenshot-audio#171.

Hosted Windows Arm64 CI now compiles the complete library and runs 516 tests; 512 pass. The remaining four failures are isolated dependency-runtime follow-ups (FFmpeg 9 spherical metadata and ImageMagick 7 ImageWriter crashes), documented on the mirror review PR: yeelam-gordon#1. The copilot-pr-autopilot round-cap circuit breaker stopped further scope expansion after all Arm64 lane/build blockers were resolved.

Latest hosted Windows Arm64 evidence: the complete library, Python binding, FFmpeg 9/OpenCV 5 compatibility code, examples, and all 516 test executables compile successfully. CTest passes 512/516. Remaining explicit follow-ups are two FFmpeg 9 spherical-metadata round-trip tests and two ImageMagick 7 ImageWriter crashes. The automated review loop stopped at its round-cap circuit breaker with zero open threads rather than hiding or disabling these dependency-runtime failures.

This branch has been narrowed to the Arm64-specific readiness surface only:

• hosted/private Arm64 CI wiring
• exact CLANGARM64 package lock
• recursive architecture validator + tests
• native Windows process-oracle coverage

The non-Arm64-specific compatibility fixes that were previously bundled here have been split into a separate prerequisite PR so this review stays aligned with the Windows Arm64 scope: #1096

Local validation on the current host:

• python ci\\test_validate_arm64_architecture.py -> pass
• python ci\\validate_arm64_architecture.py -> pass on this AMD64 host (native_machine=AMD64, so this is not a native-Arm64 runtime claim)

Thank you for separating the ARM64 build scaffolding from the broader toolchain changes! I've retargeted this PR to arm64-support. The audio changes are integrated on the matching branch first, and we'll now combine and verify the library changes before moving to the application.

We'll handle the small integration fixes here: compatibility with our GitLab version, explicit audio dependency selection, and the shared architecture/package validation improvements. #1096 remains separate until its existing review blockers are resolved, so this merge accepts the build scaffolding rather than claiming a complete native ARM64 build. Nothing is being merged into develop yet.