chore(tools): Report regex capture text in fs_modify_file

[JeanMertz]

When a regex pattern applied by fs_modify_file succeeds, the tool now reports the distinct text each match actually bound to, with occurrence counts, most frequent first. This is the one thing a regex author can't otherwise see: the pattern says what was intended and the diff says what resulted, but neither says what the engine bound. A quantifier that runs past its intended boundary (e.g. [a-z-]+ swallowing part of the delimiter-separated text that follows it) now shows up as an unexpected captured string before anyone has to read a diff hunk.

Literal patterns match themselves, so they report nothing new: a lone successful literal pattern still produces an empty report. Multi-pattern and failure reports gain a new section listing what each regex pattern matched, capped at 8 distinct entries per pattern to avoid flooding the output.

Internally, Content::replace_regexp now returns a Replacement carrying the matched strings alongside the new content, and PatternOutcome::Applied carries a matches: Vec<MatchTally> field.