feat: add Rust binary calculator example

[bryan-anthropic]

Summary

• First example of the binary server type — a compiled Rust MCP server packaged as an MCPB
• Based on the official Rust SDK calculator example (rmcp 1.3.0, 2 tools: sum/sub, ~80 LOC)
• Includes Cargo.toml, Dockerfile for Docker-based builds, .mcpbignore to exclude source from the bundle, and README with build/pack/test instructions

Motivation

All existing examples use interpreted runtimes (Node.js, Python/UV). This fills the gap for compiled languages (Rust, Go, C++) where a pre-built binary is packaged directly. The calculator is intentionally minimal — zero API keys, no external dependencies, two tools — so it serves as a clear template.

What's included

File	Purpose
manifest.json	v0.3 manifest with server.type = "binary"
Cargo.toml	Standalone Rust project (edition 2024, rmcp 1.3.0)
src/main.rs	Combined calculator server (~80 LOC)
Dockerfile	Build inside Docker (no local Rust needed)
.mcpbignore	Excludes source, keeps only manifest + binary in bundle
README.md	Build, pack, and test instructions

Cross-platform test results

Cross-compiled from WSL2 (Linux) using cargo-xwin (Windows) and cargo-zigbuild (macOS):

Platform	Claude Desktop	Result
Windows x86-64 (PE32+)	Windows CD	PASS
macOS ARM64 (Mach-O)	macOS CD (personal)	PASS
macOS ARM64 (Mach-O)	macOS CD (managed/MDM)	BLOCKED — Gatekeeper rejects unsigned binary

The managed Mac failure is OS-level security (Gatekeeper + MDM), not a CD or MCPB bug. Binary MCPBs on enterprise Macs require Apple Developer ID codesigning + notarization. This is inherent to server.type: "binary" and should be documented.

Related: This example also serves as a test fixture for #209, which adds validation and pack-time enforcement of executable permissions on binary entry points.

Test plan

• mcpb pack examples/calculator-rust validates manifest and produces correct bundle (only manifest.json + server/mcp-calculator)
• Binary built and tested end-to-end: initialize, tools/list, sum(3,4)→7, sub(10,3)→7
• yarn lint and yarn test pass (218 tests)
• Successfully tested as installed MCPB in Claude Desktop on macOS ARM64 and Windows
• ZIP stores 0755 on binary entry point — verified via external_attr inspection
• Cross-compilation viable from Linux → Windows (cargo-xwin) and macOS (cargo-zigbuild)

🤖 Generated with Claude Code

[bryan-anthropic]

Review by Claude Code

Good first example of the binary server type — clean manifest, minimal Rust code, proper .mcpbignore to keep source out of the bundle.

One minor observation: Missing schema description on SumRequest.b — SumRequest.a has #[schemars(description = "the left hand side number")] but b has no description. SubRequest has descriptions on both fields. Consider adding for consistency.

Otherwise: manifest is correct (v0.3, binary type, ${__dirname} variable, darwin/linux platforms), .mcpbignore excludes the right files, and the README build/test instructions are clear.

[tobinsouth]

Good first binary server-type example. Manifest is clean (v0.3, ${__dirname} path, correct .mcpbignore), Rust code is minimal and idiomatic rmcp. The missing schemars description on SumRequest.b is a trivial nit, not blocking. Note: the .gitignore hunk at line 88 is identical to #206's — whichever lands second will have a trivial conflict resolve.