fix(arborist): match allowScripts keys for local paths

[martinrrm]

Summary

Fixes allowScripts matching for local file and tarball dependencies, particularly on Windows.

Arborist stores a resolved Windows file source in this form:

file:C:\project\vendor\pkg.tgz

However,  npm-package-arg parses an equivalent policy key into:

saveSpec: file:C:/project/vendor/pkg.tgz
fetchSpec: C:\project\vendor\pkg.tgz

The existing matcher compared node.resolved only with saveSpec and fetchSpec. Neither comparison could match because one uses different separators and the other lacks the file:  prefix.

Changes

• Match local file and directory sources against file:${parsed.fetchSpec}, which is the same representation produced by Arborist's consistentResolve().
• Resolve relative allowScripts file keys from the Arborist project root rather than the process working directory.
• Reuse the Arborist file/directory matcher in allow-scripts-writer, keeping runtime enforcement and policy updates consistent.
• Preserve the existing exact matching behavior for remote URLs.
• Preserve deny-wins behavior when an existing local-file policy entry is false.

The implementation intentionally does not globally normalize path separators. On POSIX systems, a backslash can be a literal filename character, so replacing every  \  with  /  could cause two different sources to share an approval.

The change also does not modify dependency resolution, lockfile generation, fetching, linking, or consistentResolve().

Security considerations

Local file and tarball dependencies continue to require an exact source identity.

This change does not:

• Match local packages by their self-reported name or version.
• Case-fold paths.
• Match by basename.
• Resolve paths through realpath().
• Treat remote URLs as local file sources.
• Change registry or Git identity matching.

This preserves the manifest-confusion protections in allowScripts while recognizing the exact Windows representation npm already generates.

Testing

Added regression coverage for:

• Backslash and forward-slash absolute Windows keys matching the same local tarball.
• Relative keys resolving from the project root.
• versionedKeyFor() producing a key accepted by isScriptAllowed().
• Different local paths remaining unmatched.
• Existing false file entries continuing to block approval.
• Windows UNC paths matching through the same file:${fetchSpec} representation.
• POSIX filenames containing literal backslashes remaining distinct.
• Remote URL policies not matching file:  sources.
• npm install-scripts prune retaining valid local-file entries.

The focused Arborist matcher, policy-writer, and prune tests pass locally. Native Windows path and UNC behavior is covered by platform-specific tests and will run in the repository's Windows CI matrix.

Fixes #9900