Automate shader compiler releases

[tdecroyere]

Automate the remaining release maintenance for shader-compilers-bin.

Changes:

• publish generated releases immediately once all build jobs succeed
• avoid rebuilding a release for workflow/README-only changes
• limit the expensive DXC pull-request build to DXC/build-workflow changes
• carry the latest Metal Shader Converter packages forward into future releases automatically
• add a manual Import Metal Shader Converter workflow for the one part Apple still requires a logged-in download for
• create a reusable draft release inbox (metal-shader-converter-source) on the first import run
• accept the official Apple macOS and Windows packages uploaded to that inbox
• extract/normalize ZIP/PKG/DMG and ZIP/MSI/EXE installer forms into Elemental's include/ + lib/ layout
• validate that the macOS library contains arm64
• upload the normalized packages to the latest published compiler release (or an explicitly selected release)
• remove the temporary Apple installer assets from the inbox after a successful import
• document the flow in the README

The only remaining human step is downloading the two Apple packages while authenticated to Apple Developer and uploading them to the draft inbox. Elemental consumption remains manual.