feat(cli): wire puff check to compiler

[ofabiodev]

Summary

Wire puff check to the compiler pipeline so Puff projects can be validated directly from the CLI.

Changes

• Load and check the nearest Puff project without generating output files
• Print human-readable diagnostics to stderr and support documented JSON output with --json
• Add CLI coverage for valid, invalid, nested, missing-config, JSON, and unexpected-argument scenarios

Testing

make check
cd src
go test -count=1 ./...
go test -count=20 ./internal/cli
go build ./...