ci: share one platform matrix and deploy snapshots once

[devopvoid]

Stacked on #304, which is stacked on #302. It targets #304's branch and should be retargeted to main once both merge.

Why

The seven platform builds were defined twice, in build.yml and in release.yml, and ran through four composite actions that were copies of each other: build and release, each with a macOS Intel variant. A cache key or runner change had to be made in up to six places. On main, each of the seven jobs also deployed the whole reactor as a snapshot, racing on the snapshot metadata.

Changes

• platforms.yml (new, reusable) holds the only 7-platform matrix. build.yml calls it for pushes and PRs. release.yml calls it with the tag it releases.
• test-natives tests all 7 platforms, each on a runner of its own OS and architecture, against the natives the build job uploaded. Those are exactly the ones that get published. The macOS Intel natives now run on macos-15-intel, a real Intel runner, instead of under Rosetta on the Apple Silicon runner that cross compiles them. Before, only the 3 ARM platforms were tested this way, and the other 4 ran their tests inside the build job.
• actions/build is now the only build action.
  • The Maven profile is a matrix column, replacing the if/elif chains and the separate macOS actions.
  • It only builds. It uploads natives-<platform> and natives-media-<platform> on every platform, runs no tests, and no longer deploys.
  • actions/release, actions/build-macos-x86_64 and actions/release-macos-x86_64 are deleted.
• actions/publish (new) downloads the natives of all platforms, checks that all 7 are there, installs the host jar and runs mvn deploy -pl .,webrtc,webrtc-java-media -Dnatives.dir=…. Both publishing jobs use it.
• deploy-snapshot (new, in build.yml) makes one snapshot deployment from main with every platform's natives attached. It runs after every platform has been built and tested, the same way publish-central publishes a release. It fails if main isn't at a SNAPSHOT version.

Behavior changes

• Releases now test every platform on its own hardware before publishing. Before, release natives for the ARM platforms were never tested on ARM hardware.
• The x64 tests start after all 7 builds finish, instead of inside their own build job, so a run takes a few minutes longer.
• macos-15-intel is GitHub's last Intel macOS image, so that lane will need a new home when GitHub retires it.
• Snapshots are deployed only from main. A manually started Build of another branch no longer deploys.
• Check names change, for example platforms / build (linux_arm). main has no required checks, so nothing breaks.

Testing

• The workflows pass actionlint 1.7.12, which also checks the inputs passed to the reusable workflow. All YAML parses.
• The first commit passed a dispatched Build (36489940026) and dry-run Release (36489944490). A second pair, dispatched after the test change, covers the 7 test lanes and the release publish path. deploy-snapshot only runs on main, so it's first exercised by the first push after merge.