Adopt minimal one-shot public API

[unitmatrix]

Summary

• reduce libzget to the one-shot get/list API plus error and version helpers
• resolve member names through UTF-8, Info-ZIP Unicode Path, or CP437 and use the same semantic names for lookup
• expose compact Central Directory metadata with UTC Unix mtime resolution
• adapt the CLI, documentation, package consumers, and regression coverage

Verification

• full Debug suite: 6/6 passed
• ASan/UBSan suite: 6/6 passed
• Release AppleClang build passed
• installed CMake shared and static consumers built and ran
• shared library exports only the four intended public zget symbols

This is an intentional pre-1.0 API/ABI break for the next minor release.