fix: normalize root to always include trailing slash before path replacement - #1381

Closes #1380

Context

root: "client" and root: "client/" refer to the same directory but produced different results — the former yielded .//src/index.js because byRoot did a naive string replacement without ensuring the matcher ended with /.

Changes

• filter.go — normalize matcher to always have a trailing / before the prefix check and replacement in byRoot
• filter_test.go — fix test expectations that encoded the broken double-slash behavior; path: "folder" and path: "folder/" now both produce ./subfolder/0.rb

// before
strings.Replace("client/src/index.js", "client", "./", 1) // → ".//src/index.js"

// after (matcher normalized to "client/")
strings.Replace("client/src/index.js", "client/", "./", 1) // → "./src/index.js"