Define rigid admissible sites and G-ringed spaces

[katobungen]

Summary

• define admissible G-sites via pretopologies and their generated Grothendieck topologies
• define G-ringed and G-locally ringed spaces with sheaf-valued structure sheaves and colimit stalks
• define continuous morphisms and their induced local stalk maps
• add rigid-space saturation axioms and admissible affinoid atlases following Chapters 4–5
• keep the implementation split into focused production modules with direct root imports

Closes #35.

Verification

• ./scripts/check_root_imports.sh
• ./scripts/check_challenge_development.sh
• lake build Rigid