executeExport no longer either/or gates user data on needs_hdd — a needs_hdd
app bundles BOTH its HDD mounts and its named volumes (sonarr_config = the
whole app DB was silently dropped pre-fix). exportHDDData returns error and
fails LOUDLY on a basename collision between mounts (the manifest keys tars by
basename; the old code silently overwrote the first tar). EstimateExport made
additive to match, so the fits-on-dest gate counts both. Round-trip placement
test proves a userdata tar restores to <HDD_PATH>/userdata through the
untouched import mapping. Red-proofs recorded: either/or revert fails
scenario A; collision-check removal fails the collision test.