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Sub-slice 4b of #438. Implements `lockfile_to_hoisted_dep_graph(lockfile, opts) -> LockfileToDepGraphResult` — the walker that consumes the Slice 3 hoister output and produces the directory-keyed graph + hierarchy + hoisted_locations + direct-deps-by-importer-id + injection-targets-by-dep-path that 4a (#478) pinned the type shape of. Single-importer only (multi-importer lockfiles surface as `HoistError::UnsupportedWorkspace` via the underlying hoister). ## What's deferred Three things are intentionally left to follow-ups so 4b stays focused: - **Store fetch wiring** — `fetching` / `files_index_file` on the graph node are still defaulted. 4c will wire `StoreController::fetchPackage` (and the skip-fetch optimization that consults `currentHoistedLocations`). - **Installability check** — the walker honors `opts.skipped` as input but doesn't run `packageIsInstallable` to add new entries. 4c will plumb `package_is_installable` from `pacquet-package-is-installable` (already used by the isolated linker path). - **`prev_graph` diff** — `prev_graph` is `None`. 4d will walk the *current* lockfile alongside the wanted one (using `force: true` + empty `skipped` per upstream) and produce the orphan-removal input Slice 5's linker consumes. ## Two-pass design Upstream's `fetchDeps` walks asynchronously via `await Promise.all(deps.map(async (dep) => { ... }))`. The key invariant: each sibling's async function runs its sync prologue (insert + push to `pkgLocationsByDepPath`) before any continuation fires, because `Promise.all` sync-invokes each function and the inner `await fetchDeps(...)` only yields after the prologue completes. So by the time any node's post-recursion `graph[dir].children = getChildren(...)` line runs, every node in the entire tree is already in `pkgLocationsByDepPath`. Pacquet runs synchronously. To preserve that invariant cleanly: 1. **Pass 1 (recursive walk).** Insert each node into the graph with `children: BTreeMap::new()` and push its dir to `pkg_locations_by_dep_path`. Recursion order matches upstream's outer body (insert → push → recurse → push to `hoistedLocations`). 2. **Pass 2 (`fill_children`).** Iterate every graph node and resolve `children: alias → dir` from the snapshot's `dependencies` + `optionalDependencies` via `SnapshotDepRef::resolve`, consulting the now-complete `pkg_locations_by_dep_path`. A single-pass walker that computed children inline produced incorrect `children` maps for hoisted transitive deps — the canonical case `root → a → b` with `b` flattened to root left `a.children["b"]` empty because `b`'s pkg_locations entry hadn't been recorded yet. Caught by `walker_transitive_dep_flattens_under_root` before the refactor. ## Tests - `walker_empty_lockfile_produces_empty_result` — empty importer → empty graph, with the `"."` root importer key still present. - `walker_single_root_dep_emits_one_node` — `root → a`, node at `<lockfile_dir>/node_modules/a`. - `walker_transitive_dep_flattens_under_root` — `root → a → b`: `b` hoists to root, `a.children["b"]` points at the root-level dir. - `walker_version_conflict_keeps_loser_nested` — `root → {a@1, c}` with `c → a@2`: `a@1` at root, `a@2` nested under `c`; `hoisted_locations` records both directories; `c.children["a"]` points at the nested copy. - `walker_honors_pre_skipped_dep_path` — depPaths in `opts.skipped` are dropped from the walk entirely (and not recorded in `hoisted_locations`). - `walker_records_directory_resolution_as_injection_target` — `directory:` resolutions populate `injection_targets_by_dep_path` for the post-install re-mirror pass.