v1.8.0

Support multiple concatenated sources in $file entries

$file.src accepts a list of sources like $dir.src: contents are
concatenated in order (a newline is inserted between parts when one is
missing), and list entries can carry a per-source onlyWhen gating just
that section. When every source is gated off, no file is generated and
a previously written untouched file is removed on sync. State records
the space-joined active sources and +-joined blob ids, matching the
$dir combined-key scheme and staying backward compatible for
single-source files.