feat(validator): integrate gp-module offline preflight validation

[ValgulNecron]

Summary

Integrates gp-module validate --json into validate.py as an offline preflight check for module manifests while preserving backward compatibility.

• Discovers gp-module binary in PATH or adjacent build directories.
• Adds --offline flag to run static preflight checks directly.
• Aggregates offline rule diagnostics with line numbers alongside existing registry and format checks.