fix(init): clean up stale dtvem shims entries from PATH

[CalvinAllen]

Summary

When XDG_DATA_HOME is set on Windows/macOS, dtvem honors it for data storage and PATH config. But users who upgraded from a pre-XDG version (or set XDG_DATA_HOME after a prior install) end up with a stale ~/.dtvem/shims entry in PATH pointing at an empty directory, while the real shims live at the XDG-resolved location. dtvem init previously only managed the path it currently resolves to and didnt notice the stale leftover.

This PR teaches dtvem init to:

• Detect entries in PATH that look like a dtvem shims directory (*/dtvem/shims or */.dtvem/shims, case-insensitive on Windows) but dont match the currently-resolved ShimsDir().
• On Windows: remove them from the registry during the existing PATH-modify pass; log each removal in the success output.
• On Unix: warn the user with manual cleanup instructions (we dont auto-rewrite .bashrc/.zshrc because users often customize them heavily).

What this fixes

The original bug from #247:

• XDG_DATA_HOME set, real shims correctly created at ~/.local/share/dtvem/shims
• PATH contains stale ~/.dtvem/shims entry (empty dir) from a prior install
• dtvem init previously didnt touch the stale entry

After this PR, running dtvem init (then restarting the terminal) on a machine in this state will:

• Remove the stale ~/.dtvem/shims entry
• Add the correct ~/.local/share/dtvem/shims entry
• Print which stale entries were removed

Implementation notes

• New cross-platform helpers in src/internal/path/path.go:
  • IsDtvemShimsPath(path) — matches both dtvem/shims and .dtvem/shims parents (case-insensitive on Windows)
  • FindStaleShimsEntries(entries, currentShimsDir) — returns entries that look like dtvem shims dirs but arent the current one
  • SplitPath(pathEnv) — splits PATH using the OS-appropriate separator
• path_windows.go:
  • checkSystemPath / checkUserPath now flag stale entries as needing an update even when the current shimsDir is already at position 0
  • modifySystemPath / modifyUserPath filter out stale entries during the registry rewrite and log each removal
• path_unix.go:
  • AddToPath now calls warnAboutStaleShimsEntries to surface stale entries with manual cleanup instructions

Test plan

• ./rnr check passes (lint + all tests)
• New unit tests cover IsDtvemShimsPath (incl. Windows case-insensitivity) and FindStaleShimsEntries (incl. .dtvem vs dtvem parents, ordering preservation, Windows case-insensitive comparison)
• Manual verification on a setup with both ~/.dtvem/shims and ~/.local/share/dtvem/shims in PATH: run dtvem init, confirm stale entry is removed and current entry is added
• Verify Unix warning output matches expectation when a stale entry is in PATH

Resolves

Resolves #247