…stall dir

`run_global` calls `std::fs::canonicalize` on the freshly-created install
directory and on `layout.pkg_dir`. On Windows those calls return the
extended-length form `\\?\C:\…`. Every downstream step that concatenates
this dir into another path then carries the verbatim prefix forward, and
the bin-shim writer in particular ends up baking it into the relative
target string it interpolates into `%~dp0\{rel}`. The resulting `.cmd`
points at `<bin_dir>\\\\?\\C:\…\prettier.cjs`, which `cmd.exe` collapses
to `<bin_dir>\?\C:\…\prettier.cjs`, which Node then refuses with:

    Cannot find module 'C:\…\bin\?\C:\…\node_modules\prettier\bin\prettier.cjs'

There is already an inline fix for the same class of bug for the per-dep
bin dir at `aube::commands::install::link_bin_via_dep`. This PR generalizes
it into a reusable `dirs::canonicalize` helper (matches `dunce::canonicalize`
without adding the dep) and routes the three `run_global` call sites
through it so the install dir, the layout pkg dir, and the symmetric
post-install equality checks all see the same plain drive-letter form.

Reproduced via mise's `windows-e2e` job:
https://github.com/jdx/mise/actions/runs/24852804095 (the
`mise install npm:[email protected] -- prettier --version` step).

Validation:
- `cargo test --bin aube` — 281 pass (1 new, `canonicalize_round_trips_an_existing_path`;
  also a `#[cfg(windows)]` `canonicalize_strips_verbatim_drive_prefix`)
- `cargo fmt --check`
- `cargo clippy --all-targets`

Signed-off-by: jdx <[email protected]>

… dirs::canonicalize

Two follow-on fixes to the same Windows verbatim-prefix class of bug as the
parent commit:

- `global::scan_packages` (and via it, `find_package`) populated
  `GlobalPackageInfo.install_dir` from `std::fs::canonicalize`, so the
  field still carried `\\?\C:\…`. The new `run_global_inner` prior-cleanup
  code compares those paths against `install_dir` (now stripped via
  `dirs::canonicalize`) at `add.rs:1064-1077`. Without this fix the
  comparisons never match on Windows and stale install dirs leak every
  reinstall of the same alias set. `remove_package` had the same shape —
  `pkg_canon` was verbatim, but `info.install_dir` is now stripped, so the
  `starts_with` guard would fail and the install dir would never be
  removed.

- `commands::install::bin_linking` carried an inline `s.strip_prefix(r"\\?\")`
  that the parent commit's helper was extracted from. The inline version
  unconditionally strips the prefix, including from real `\\?\UNC\server\share`
  paths that have no non-verbatim equivalent. Replacing it with
  `dirs::canonicalize` (which preserves UNC share paths) keeps the
  behavior identical for the only path it ever sees in practice today
  (drive-letter junctions) and removes the only divergent copy of this
  logic.

Greptile flagged both during review of #243 (P1 + P2).

Validation:
- `cargo test --bin aube` — 281 pass
- `cargo fmt --check`, `cargo clippy --all-targets`

Signed-off-by: jdx <[email protected]>