Fix deadlock, silent data loss and crashes in the sort pipeline

[lanrat]

• Run the build, sort and save stages in one errgroup, so a save error (toBytes failure, full disk) no longer deadlocks Sort() and a sort error no longer leaks the save goroutine and its temp file.
• Report a read error on a chunk's first record instead of silently dropping the chunk, and treat a length header without its payload as io.ErrUnexpectedEOF instead of a clean end of chunk.
• Return the recovered error from the legacy FromBytes wrapper (named results) instead of (nil, nil).
• Convert panics in compareFunc, fromBytes and toBytes during the save and merge stages into ComparisonError, DeserializationError and SerializationError instead of crashing the process.
• Close the temp reader before closing the result channels, fixing the "send on closed channel" panic when Close fails.
• Create the temp file lazily, once a second chunk exists, and close it on every path. Creation errors are reported on the error channel, so the constructors never return a nil sorter.
• Release the .extsort_ directory reference when the reader closes.
• Temp dir selection: return an explicit TempFilesDir unchanged so New reports it when unusable, and skip default candidates that are missing or not writable (no /var/tmp, read-only root) instead of failing.

Add regression tests for each bug, and make TestDeserializationError, TestNilInputs and TestComparisonFunctionPanic reach FromBytes and the merge and require an error.