feat(size-report): replace savings log with status reports

Rename the old savings log feature to size report across the CLI,
configuration, documentation, man page, and bash completion. The new
`--size-report` option keeps the existing `-s` shorthand, while
`--saving`, `--saving-file`, and the `[saving]` config section are
replaced by `--size-report`, `--size-report-file`, and `[size_report]`.

Extend reporting beyond successful encodes. The TSV log now gains a
leading `status` column and records encoded files, codec skips,
only-if-larger skips, and failed inputs. Size-report output also prints
per-file feedback for encoded and skipped-larger files, including
human-readable original/new sizes, bytes saved, and saved percentage.

Track batch-level report counters and print an end-of-run summary for
multi-file runs with encoded/skipped/failed counts, total bytes saved,
and the overall weighted saving percentage across successful encodes.

Document the new TSV schema in the README and man page, update examples
and config snippets, and adjust bash completion for the renamed options.

BREAKING CHANGE: `--saving` is now `--size-report`. `--saving-file` is
now `--size-report-file`. `[saving]` in config.toml is now
`[size_report]`. The TSV log gains a leading `status` column and may
contain skipped and failed rows in addition to successful encodes.

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