fix(sed): preserve escaped replacement characters

[Voyagerroc-Lab]

shx sed silently corrupts escaped replacement characters. For example, replacing foo with the escaped Windows path C:\\temp\\file.txt produces C:.temp.file.txt and exits successfully. This also affects -i, writing the corrupted path to disk.

Unescape replacement characters in one pass, retaining the escaped character instead of substituting a dot. A single pass also prevents adjacent escaped backslashes and slashes from being interpreted twice.

Regression tests exercise file input without -g, stdin with adjacent escapes, and in-place global replacement. They verify output/file contents, exit status, and stderr. All three fail on unchanged main (3d4e06c49a4f4835b9f9aedc728a71d6b7b06493) and pass with the fix.

Validation on Windows, Node v24.19.0:

• Invoked src/cli.js as a child process with piped input before and after the fix; confirmed both the Windows path and adjacent-backslash/slash reproductions.
• npm test: 42 passing, 3 existing platform skips for chmod; posttest ESLint passed.
• npm run check-node-support: passed.
• git diff --check: passed.

This fixes literal replacement escaping only. It does not implement Unix sed basic regex syntax or numeric backreferences, and does not close the broader compatibility request #225. Other operating systems and Node versions were not run locally.