feat(core): add batch progress counter to processing output

encode_one now receives the current index and total file count and
includes them in the "Processing" message (e.g. "Processing [2/7]:
file.mkv"). Both the CLI and encode-file paths pre-collect their file
lists into arrays so the total is known before encoding begins.

Also fix a typo in the encode_one step comment ("preserverse" ->
"preserves") and add a note on the filename variable explaining the
single-extension stripping behaviour for multi-dot filenames.

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