Copilot on #14138 flagged that the share report says '(file not found)'
when the log exists but is empty (either because the primary is empty
and no .1 rotation exists, or in the rare race where the file is
truncated between _resolve_log_path() and stat()).
- Split _primary_log_path() out of _resolve_log_path so both can share
the LOG_FILES/home math without duplication.
- _capture_log_snapshot now reports '(file empty)' when the primary
path exists on disk with zero bytes, and keeps '(file not found)'
for the truly-missing case.
Tests: rename test_returns_none_for_empty → test_empty_primary_reports_file_empty
with the new assertion, plus a race-path test that monkeypatches
_resolve_log_path to exercise the size==0 branch directly.
b641639e42