protoc assets for macOS and Linux arm64 are not selected (osx, aarch_64) #171

Description

dload get protoc fails on macOS (arm64 and x86_64) and on Linux arm64: no asset of a protoc release is selected, although the release has a build for each of these platforms. Linux x86_64 and Windows work.

protoc names its assets with tokens DLoad does not recognize:

protoc-36.2-linux-aarch_64.zip
protoc-36.2-linux-x86_64.zip
protoc-36.2-osx-aarch_64.zip
protoc-36.2-osx-universal_binary.zip
protoc-36.2-osx-x86_64.zip
protoc-36.2-win32.zip
protoc-36.2-win64.zip

• osx is not an OS token: OperatingSystem::tryFromString() knows darwin and macos only, so every osx-* asset is dropped for Darwin.
• aarch_64 is not an architecture token: Architecture::tryFromBuildName() matches aarch64 but not the underscored form, so linux-aarch_64 and osx-aarch_64 are dropped for arm64.

Steps to reproduce

1. DLoad 1.19.0, <download software="protoc" version="^36.0"/> in dload.xml (or vendor/bin/dload get protoc).
2. Run on macOS or Linux arm64 (or force the platform the way roadrunner/cli does: os darwin / linux, arch arm64).
3. No protoc asset is selected and the download fails.

Proposed solution

• OperatingSystem: read osx as Darwin (in tryFromString() and in the build-name pattern of tryFromBuildName()).
• Architecture: read aarch_64 as ARM_64 (aarch[_-]?64, like the existing x86[_-]64).
• Optionally treat universal_binary / universal as a macOS build for both architectures, as a fallback when no arch-specific asset exists.
• A test over the real protoc asset list above.

Additional context

Blocks rr get-grpc-tools (roadrunner-php/cli#76) on macOS and Linux arm64: buf and protoc-gen-php-grpc install there, protoc does not. spiral/roadrunner-bridge 5.x generates gRPC code with buf and needs protoc next to it.