ImportBase opened the input with a default std::fstream (in|out), which had
two consequences: a missing file silently produced an empty module (no error),
and a present-but-read-only file failed to open and also loaded as empty. Open
the stream read-only (std::ios::in) instead, and expose is_open().
System::Load now builds the importer first, checks is_open(), and throws
"cannot open file: <path>" before creating the module — so a failed load
surfaces as `load failed: …` and leaves no empty module behind. A read-only
but present file now loads correctly.
Flip the test that pinned the old silent-empty behaviour to assert the clean
failure (error + no module created).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
François
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