Add MCPB distribution bundle

[dweinstein]

What & why

• add one installable MCPB containing a universal macOS binary and Windows amd64 binary
• let installers select Platform or MARI through the required --product parameter
• replace the previous --platform and --mari interface with --product platform|mari
• build and verify MCPB artifacts in CI and attach versioned bundles to tagged releases
• document Claude Desktop installation and local bundle builds

Validation

• go test ./...
• go vet ./...
• official MCPB manifest schema validation
• bundle ZIP, SHA-256, universal-binary, code-signature, and Windows payload checks
• MCP initialize and tools/list smoke tests for both products

Self-review

• Tests added/updated
• Ran it locally
• Reviewed own diff
• Edge cases considered