dual billing; tracker to node model sharing

docs/issues/44-tracker-shard-source-partial-download.md

@@ -1,6 +1,6 @@
 # US-044 — Tracker as model-file source; nodes download only their shard
 
-Status: planned
+Status: in progress
 Priority: High (blocks multi-machine big-model serving; pairs with US-042)
 Stage: Designed (grill remaining decisions before build)
 
@@ -56,10 +56,30 @@ What exists already (build on it, don't duplicate):
 
 ## Acceptance criteria
 
-- Two-machine test: machine A (tracker + node, holds full snapshot) serves
+- [x] Tracker can be started with `--models-dir PATH` / `MESHNET_MODELS_DIR`
+  and advertises a local model-file source in assignment responses when it has
+  a matching HF snapshot.
+- [x] Tracker serves a tar stream containing only the safetensors files selected
+  for the assigned layer range plus config/tokenizer/index metadata.
+- [x] Node downloader keeps exact-shard peers first, then races tracker model
+  sources against a HuggingFace `snapshot_download(..., allow_patterns=...)`
+  subset download, using the first successful source.
+- [x] Real PyTorch model startup can use tracker `full_url` sources to fetch
+  the full local snapshot over LAN before `from_pretrained`, so local-network
+  testing no longer has to pull from HuggingFace first.
+- [ ] Two-machine test: machine A (tracker + node, holds full snapshot) serves
   layers 0–k; machine B joins with no model and receives **only** the files
   for its assigned range from A — nothing fetched from HF
-- Machine B's resident memory scales with its shard size, not model size
-- Checksums verified end-to-end; corrupted transfer falls back cleanly
-- Single-node/full-model flows unchanged
-- `python -m pytest` passes from repo root
+- [ ] Machine B's resident memory scales with its shard size, not model size
+- [ ] Checksums verified end-to-end; corrupted transfer falls back cleanly
+- [x] Single-node/full-model flows unchanged
+- [ ] `python -m pytest` passes from repo root
+
+## Implementation notes
+
+- 2026-07-06: Added the tracker/node download path. For immediate Qwen3.6-35B
+  LAN testing, real PyTorch nodes fetch the full snapshot from the tracker via
+  `full_url` and race HuggingFace as fallback. Remaining hard half is true
+  partial model materialization: the backend can prefer a downloaded local
+  model directory, but Transformers still needs a `meta`-device load path that
+  materializes only assigned layers.