Masks non-empty '...' content before T5/T2-T4 run so literal strings such
as `echo '${IFS}'` are not rewritten. Empty '' pairs are stripped first
so c''u''rl -> curl evasion keeps resolving. ANSI-C $'...' is decoded
before masking.
Caught by the false-positive probe added in Step 3 of ultraplan-v6.2.0.