Partial standard-library cache directories are treated as complete packages

[aspizu]

Repro

home=$(mktemp -d /tmp/goboscript-home-XXXXXX)
tmp=$(mktemp -d /tmp/goboscript-std-partial-XXXXXX)
printf 'std = "2.1.0"\n' > "$tmp/goboscript.toml"
printf 'costumes "blank.svg";\n%%include std/math\n\n' > "$tmp/stage.gs"
printf '<svg xmlns="http://www.w3.org/2000/svg" width="1" height="1"></svg>\n' > "$tmp/blank.svg"
mkdir -p "$home/Library/Application Support/com.aspizu.goboscript/std/v2.1.0"
HOME="$home" target/debug/goboscript build "$tmp"

Observed:

error: No such file or directory (os error 2)
 --> .../stage.gs:2:10
2 | %include std/math
  |          ^^^^^^^^

fetch_package() returns immediately when the version directory exists, even if the directory is empty or only partially extracted. The compiler then treats the broken cache as the installed std package.

Expected

The std cache should be validated before reuse, or package extraction should be atomic so partial directories are not accepted as complete packages. A broken cache should be repaired or reported as cache corruption, not surfaced as a missing %include file in user code.

[linear]

GS-282