Fix regex preview (-c)

[IAKOBVS]

This commit fixes the interactive regex replacement preview (-c) by implementing 10-match capture in confirm_scan_file, shifting subgroup offsets to be absolute, and introducing a custom regex_expand_rplc helper that performs on-the-fly backreference expansion. It also fixes grep assertions in confirm.sh and registers a new test for global and greedy regex previews.

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PR created automatically by Jules for task 8424390211604614302 started by @IAKOBVS

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