Per-importer hoisting roots for nodeLinker: hoisted (multi-level output)

Close the last structural gap between pacquet's @yarnpkg/nm hoister port (pnpm/crates/real-hoist) and upstream: per-importer hoisting roots. Today nm_hoist runs hoist_into_root against the single virtual . root; upstream hoistTo recurses into every node marked as a hoist root (workspaces under hoistingLimits: 'workspaces', direct deps under 'dependencies'), producing a multi-level tree where each bordered subtree gets its own fixed-point hoist.

What works today (and must not regress)

• Non-root importers are attached to the tree unconditionally (v11 parity, pnpm/pnpm#12899), so cross-project version dedupe and conflict nesting work: a conflicting version stays nested at its position in the subtree and the walker (hoisted_dep_graph.rs::walk_deps) materializes it there.
• Hoisting borders are honored one level deep: a name in the root locator's hoisting_limits set blocks its descendants from hoisting past it (real-hoist/src/lib.rs::hoist_subtree, under_border).
• hoist_workspace_packages name-links (v11's hoistedWorkspacePackages) are a separate, implemented shape — they do not depend on this work.

What multi-level adds

Within a bordered subtree (e.g. a workspace importer under hoistingLimits: 'workspaces'), upstream still hoists internally: the importer's transitive deps flatten up to the importer's own node_modules instead of staying at their natural tree depth. Pacquet leaves them nested. Nested placement is resolution-correct (Node walks up), so the user-visible difference is layout/dedupe density, not resolvability — this is why the gap has been shippable so far.

Sketch

1. get_hoisting_limits already emits per-importer border entries for the 'dependencies' mode ("become load-bearing once multi-level hoisting lands" — package-manager/src/hoisting_limits.rs). Keep that shape.
2. In nm_hoist, after the root fixed point, recurse: for each surviving child that is itself a hoist root (Workspace-kind, or name in a border set with a per-locator hoisting_limits entry), run hoist_into_root with that node as the root and its locator's border set. Upstream reference: hoistTo at yarnpkg-nm/sources/hoist.ts:329 and its hoistIdents/per-root preference maps.
3. The walker (walk_deps) needs no structural change — it already recurses into Workspace-kind children and places whatever the hoister decided.
4. Tests to port: upstream hoist.test.ts cases for hoistingLimits ('workspaces' and 'dependencies') and the CLI known-failure partial_install_persists_hoisted_map (pnpm/pacquet#433) which also depends on re-hoist merging.

Risks / order of work

• The hoister's per-pass ident shift and preference maps are global today; per-root recursion needs them scoped per hoist root or recomputed per subtree (upstream recomputes: buildPreferenceMap(rootNode) per hoistTo).
• Do after pnpm/pacquet#433's partial-install work only if re-hoist merging is wanted in the same release; the passes are independent otherwise.