fix(macho): add LC_UUID to Darwin executables

[onevcat]

Summary

• add an LC_UUID load command to directly emitted Darwin Mach-O executables
• derive the UUID bytes deterministically from emitted text/rodata so repeated builds stay byte-for-byte stable
• cover the load command in command-contract and native-test checks

Closes #28.

Verification

• make -C native/zero-c
• zero run hello.0 now prints hello from zero on Darwin arm64
• npm run command-contracts:local --silent
• npm run native:test:local --silent

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[ctate]

Hey @onevcat - I appreicate issue #28 and this PR

Merged a fix in #55 with you as a co-author