[tomdavidson]

Adds a TOML plugin for microsandbox (msb), installing the official prebuilt release bundle with checksum verification.

• Plugin id is msb, matching the binary name at the archive root, so no exe-path is needed.
• No darwin x86_64 build exists upstream, so macOS is limited to aarch64.

Not verified:

• Whether msb finds the versioned libkrunfw sitting next to it in proto's flat install directory. The official installer moves it to lib/ and adds symlinks. CI only checks the binary exists and is executable, so it will not catch this.
• Windows archive layout (assumes msb.exe at the root).
• Linux bundles need glibc 2.28+. The plugin cannot block musl hosts.

Manual test before merge: with ~/.microsandbox moved aside, proto install msb, then msb --version and msb run ubuntu.