fix(sed): support backreferences in search patterns

[chaliy]

Summary

• Add fancy-regex as fallback for sed patterns containing backreferences (\1-\9) in the search side
• The standard regex crate doesn't support backreferences in patterns; fancy-regex does
• Transparent SedRegex wrapper tries regex first, falls back to fancy-regex only when backreferences are detected
• No performance impact for patterns without backreferences (still uses fast regex crate)

Test plan

• Unit test: search-side backreference matching repeated characters (\(.\)\1)
• Unit test: HTML tag stripping with backreference in search pattern
• All 24 existing sed unit tests pass (no regression)
• All 15 spec test suites pass
• cargo clippy clean, cargo fmt applied

Closes #1159