ci(mac): upload macos-arm64 ffmpeg tree as one-shot artifact (universal-build prep)

Bundling-strategy iteration toward macOS universal builds. The mac-14
runner already produces libs/ffmpeg/macos-arm64/ during the existing
"Vendor LGPL ffmpeg" step, but the working tree is discarded when the
runner shuts down. Capturing it as a uploaded artifact lets us commit
it once into the repo (libs/ffmpeg/macos-arm64/), pairing with the
already-vendored libs/ffmpeg/macos-x86_64/ to unlock:

1. Local universal mac builds (CMAKE_OSX_ARCHITECTURES="arm64;x86_64")
   once cmake/ffmpeg.cmake's "warn-and-fall-back-to-host" logic at
   lines 26-29 is replaced with proper per-arch handling.
2. CI's "Vendor LGPL ffmpeg" + "Verify LGPL discipline" steps to be
   deleted entirely — pre-built bytes mean a 15-25 min CI step becomes
   a no-op git checkout. Reduces toolchain-rot surface (no more MSYS2
   package list debugging on Windows; mac arm64 source-build openh264
   no longer needs to run every CI invocation).

Once libs/ffmpeg/macos-arm64/ lands in the repo, both this upload step
AND the upstream Vendor/Verify steps can be removed. Pattern can
extend to libs/ffmpeg/windows-x86_64/ + libs/ffmpeg/linux-x86_64/ as
those CI lanes start succeeding.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mkschulze

.github/workflows/juce-build.yml

@@ -60,6 +60,20 @@ jobs:
         # on any contamination.
         run: bash scripts/verify_ffmpeg_lgpl.sh
 
+      - name: Upload vendored macos-arm64 ffmpeg tree (one-shot for bundling)
+        # Phase 23 (regression-fix scope) iteration: capture the arm64 ffmpeg
+        # vendored tree built on the macos-14 runner so we can commit it to
+        # libs/ffmpeg/macos-arm64/ locally, then skip the from-source rebuild
+        # entirely on subsequent CI runs and unlock universal-mac builds. This
+        # step is intentionally one-shot — once libs/ffmpeg/macos-arm64/ lands
+        # in the repo, both this upload step AND the Vendor/Verify steps above
+        # can be deleted (replaced by a no-op git checkout).
+        uses: actions/upload-artifact@v4
+        with:
+          name: libs-ffmpeg-macos-arm64
+          path: libs/ffmpeg/macos-arm64/
+          retention-days: 7
+
       - name: Configure CMake
         # Production-target-only config for v1.3 betas — matches beta.20.5
         # surface. JAMWIDE_BUILD_TESTS + JAMWIDE_VIDEO_SPIKE pulled the test