Summary

• Fix sed builtin to properly unescape \/ in replacement strings, producing literal /
• Also handles \\ → \, \n → newline, \t → tab in correct order using placeholder approach
• Adds spec tests for slash unescaping, back-reference groups with slashes, and escaped backslash

Test plan

• sed_unescape_slash_in_replacement — \/ produces /
• sed_regex_groups_with_slash — back-references with \/ in date reformatting
• sed_escaped_backslash_in_replacement — \\ produces \
• All existing sed spec tests still pass
• Smoke tested via CLI: echo "abc" | sed 's/b/\//' → a/c

Closes #959