feat(core): verify output integrity before replacing original

After ffmpeg exits successfully, run ffprobe against the temporary
output file to confirm it is a valid, playable media file before
replacing the original. If the probe fails the temporary file is removed
and the encode is treated as a runtime failure, leaving the original
untouched.

This guards against cases where ffmpeg exits 0 but has written a
truncated or corrupt container (e.g. due to a disk-full condition
mid-write, a codec bug, or a preset producing an unreadable stream).

The check is enabled by default and can be disabled via
--no-verify-output or [encoding] verify_output = false in config.toml
for large batch jobs on trusted encoders where the extra ffprobe
  round-trip is undesirable.

The --continue flag is honoured: a failed integrity check counts as a
runtime failure for the purposes of deciding whether to proceed to the
next file.

Docs, man page, bash completion, and README updated accordingly.

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