Refactor Build Process: Use pokesprite as submodule

[tmck-code]

Context

This PR moves the pokesprite repository and cowfile generation out of the Dockerfile to a git submodule and build script, which significantly improves the speed and error handling of the cowfile generation process.

Changes

Cowfiles build in parallel outside Dockerfile

• Move cowfile building out of dockerfile
  • Reduces Docker image size by not embedding pokesprite files directly
• Use the submodule directory instead of cloning at build time
  • Allows version control of the pokesprite dependency
• Alter the makefile task to use the new build_cowfiles.sh script

Improved error handling, Better file management and user permissions

• Improved error handling with explicit failure logging
• Deduplicates cowfiles using a set-based approach instead of linear search
• Track conversion failures and display them in debug mode
• Don't overwrite original pokesprite files
  • Use a tmp directory for intermediate files during conversion
  • Prevents pollution of the original pokesprite submodule
• Better user permissions
  • Use user u instead of root for better security
  • Remove all use of root user in local build scripts
  • Update other makefile tasks

Updated github workflow

• Build cows as a separate step
• Ensure pokesprite submodule is available during build

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stage	time before	time after
cowfile gen	47s	4s
all	2m14s	1m32s