`--help`, the module doc and the README promise that a `--memory` session
"writes nothing", and the desktop app broke that in two places:
- `PrefsWriter` saved `ui.json` whenever the UI state changed (issue #164),
so a throwaway session's `thumbSize`, window layout or language replaced
the user's saved settings — the README showcase script does exactly that.
- The GPU crash sentinel from #136 armed itself in every session except
`LIGHTCRAFT_NO_PREFS` (issue #169); `with_init_marker` creates the
settings folder to hold `gpu-init.marker`, removes the marker and leaves
the folder, so a `--memory` run on a fresh home left an empty
`Library/Application Support/LightCraft` behind.
Now `PrefsWriter` carries its target path (`None` for `--memory`,
`LIGHTCRAFT_NO_PREFS` or no home folder) and writes nothing without one; it
also writes nothing while the session is the one offered when the library
can't be opened (the question window and Continue Without Saving, which
`docs/control-protocol.md` already describes as writing nothing) and
resumes when `app.openLibrary` ends it. `gpu_crash_check(in_memory)` neither
arms the sentinel nor removes a marker in a `--memory` session; it still
reads one (new `read_init_marker`), so GPU rendering starts off there too
and the next ordinary launch reports and clears it. A `--memory` session
still reads `ui.json`, but a damaged one is reported and left in place
rather than set aside. Normal launches are unchanged.
Tests: `prefs_writer_writes_nothing_in_a_memory_or_temporary_session`,
`memory_session_leaves_the_gpu_marker_and_folder_alone`,
`memory_session_leaves_a_damaged_ui_json_alone` (apps/lightcraft), and
`read_init_marker` coverage in the gpu crate's marker test.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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