Prepare release v0.5.0

[unitmatrix]

Summary

• bump the single version source to 0.5.0
• refresh the zget(1) revision date for the updated output contract

Readiness

• README and zget(1) describe curl-compatible named output
• installation documentation uses release-version placeholders
• release workflow validates tag and binary version parity

Tests

• ctest --test-dir build --output-on-failure
• ASAN_OPTIONS=strict_string_checks=1 UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1 ctest --test-dir build-stage4-sanitize --output-on-failure