refactor(core): remove awk and bc dependencies

Both tools were used for simple arithmetic that bash handles natively.

awk: fps rounding (num/den) replaced with round-half-up integer
arithmetic: (num + den/2) / den.

bc: savings percentage replaced with bash $(( )) arithmetic. bc was a
conditional dependency (only required with --saving), which made the
dependency story inconsistent. Removing it means the full feature set is
available with only ffmpeg, ffprobe, nice, and optionally tomlq.

Updated README requirements table, man page --saving description, SEE
ALSO section, and exit-code 127 entry accordingly.

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