fix: skip isolated tree for workspace-filtered installs with linked strategy

[manzoorwanijk]

In continuation of our exploration of using install-strategy=linked in the Gutenberg monorepo, which powers the WordPress Block Editor, this is a follow-up of the fixes from #8996.

npm install --workspace crashes with a TypeError when install-strategy=linked is configured.

The isolated tree built by _createIsolatedTree() is structurally incompatible with workspace-filtered installs:

1. root.children is an Array, not a Map — workspace filtering calls .get(ws) which doesn't exist on Array
2. inventory.query() is a stub returning [] — the set root setter can't resolve Link targets during post-reify re-rooting
3. The filterSet computed against the isolated tree becomes invalid after the tree is swapped back to the original

npm 11.x throws:

TypeError: Cannot read properties of null (reading 'package')
    at set root (lib/node.js:797:35)

npm 10.x throws:

TypeError: this.idealTree.children.get is not a function
    at Arborist.[diffTrees] (lib/arborist/reify.js:441:45)

This PR:

1. Skips _createIsolatedTree() when --workspace is active, falling back to the normal reify path with real Node/Link instances. A subsequent full npm install restructures everything back to linked layout. The isolated tree was designed for full npm install, not filtered installs.

2. Handles unsupported spec types in isRegistryDependency — packages with link: specs in their dependencies (e.g. link:.) cause npa to throw EUNSUPPORTEDPROTOCOL when isRegistryDependency is accessed during lockfile save. The fix catches the error and treats unsupported spec types as non-registry dependencies.

References

Fixes #9038