Never skip a local index error for a directory or symlink (closes #284)

Under --skip-errors, an error recording a directory or symlink in the
local index was skipped like an unreadable file. The snapshot completed
without the entry, and a restore did not recreate it.

processFileWithErrorHandling now records a directory or symlink itself
and returns any error, which stops the backup with or without the flag.
The skip sees only a regular file's errors.

The test injects the error with a SQLite trigger that refuses the
directory's files row.

Model: opus-5-5

internal/snapshot/scanner.go

@@ -1406,7 +1406,12 @@ func (s *Scanner) processFileWithErrorHandling(
 
 // countFailedFile counts a file that phase 2 could not store as failed
 // and takes its size back out of BytesScanned, where phase 1 put it.
+// Phase 1 counts no directories, so a directory is not counted here.
 func countFailedFile(fileToProcess *FileToProcess, result *ScanResult) {
+	if fileToProcess.FileInfo.IsDir() {
+		return
+	}
+
 	result.FilesFailed++
 	result.BytesScanned -= fileToProcess.FileInfo.Size()
 }