feat(coreutils): install coreutils in ext2 /bin directory

[ryanbreen]

Summary

• Rename coreutil binaries from *_cmd to standard names (ls, echo, mkdir, etc.)
• Update ext2 disk creation script to install coreutils in /bin/
• Regenerate testdata/ext2.img with coreutils binaries

Changes

• Rename 7 source files: ls_cmd.rs → ls.rs, etc.
• Update Cargo.toml binary definitions
• Update build.sh BINARIES array
• Update init_shell.rs PROGRAM_REGISTRY mappings
• Enhance create_ext2_disk.sh to copy ELF binaries to /bin/

ext2 /bin/ Contents

/bin/cat      - Concatenate and print files
/bin/ls       - List directory contents
/bin/echo     - Print arguments
/bin/mkdir    - Create directories
/bin/rmdir    - Remove empty directories
/bin/rm       - Remove files
/bin/cp       - Copy files
/bin/mv       - Move/rename files
/bin/hello_world - Exec test binary (exit 42)

Test plan

• All 217 boot stages pass
• Coreutils load from ext2 /bin/ via exec syscall
• Fallback to test disk still works for test-specific binaries

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