test(mcp): prove composite capability programs

[jmgilman]

Summary

• prove composite output discovery through the catalog and unchanged flat MCP schema
• add a real same-binary MCP program that loops, filters, and aggregates []struct output
• assert canonical authorization arguments, exact typed handler input, derived notation, and one native call
• prove public registration and execution project unsupported outputs and resource limits safely
• include both secure-loop and composite-loop cases in the MCP smoke gate

Part of #23. Documentation is the final increment.

End-to-end behavior

The actual MCP client/server transport describes list[{id: str, active: bool, score: float}], executes one records.search native call, filters inactive rows in Starlark, and returns only {count: 2, score: 3.75, ids: ["keep-a", "keep-c"]}. The optional label is absent from the canonical authorization map and nil in the exact typed Go handler input.

Validation

• go test ./internal/catalog ./mcpserver .
• moon run root:mcp-smoke
• golangci-lint run --config .golangci.yml ./internal/catalog ./mcpserver . --show-stats=false

Review

Bounded persistent review found no issues. It verified actual transport and worker re-exec, canonical authorization, typed input, exact notation, digest-only output, public error projections, flat schema, catalog immutability/sorting, and the moon regex.