Only instantiate coders once

[jeffcarbs]

Summary

• Avoids creating new coder instances on every save and load call by memoizing them on the runner
• FileCache#read and FileCache#write also reuse the same instances via a private coder_for helper rather than calling .new inline
• Some opportunistic renames:
  • Cache#data -> Cache#content
  • Coder#save -> Coder#generate
  • Coder#load -> Coder#mount

Test plan

• Existing cache and file cache specs continue to pass

🤖 Generated with Claude Code