feat(hub): resume interrupted model downloads via HTTP Range (Node)

[renezander030]

Follow-up to the discussion in #1220 (thanks @emojiiii for handing it over, and @sroussey for the Xet/CDC pointer).

This is step 1: HTTP Range-resume for the Node file-cache path, which is where large model weights are streamed to disk. It ships resume for how downloads work today, with no new transport dependency. Xet-native chunk caching (via the huggingface.js download path) is the natural follow-up and is intentionally out of scope here.

What changes

• FileCache streams large downloads to a deterministic <key>.incomplete file plus a small sidecar ({ etag, total }), and keeps the partial on failure instead of deleting it.
  • getResumeInfo(request) reports { size, etag, total } when a consistent partial exists.
  • put() branches on the response status: 206 appends to the partial (offset/total parsed from Content-Range); 200 restarts cleanly (truncates any stale partial). A truncated body (loaded < Content-Length) is rejected and the partial retained for the next attempt.
  • A per-key lock preserves the previous concurrent-writer guarantee: if the partial is already held by another writer, put() falls back to the original unique-temp path (correct, non-resumable, never corrupts the shared partial). Stale locks are stolen after a TTL.
• hub.js only sends Range/If-Range on the streaming path (IS_NODE_ENV && return_path), since a 206 on the buffered path would yield only the trailing bytes. It now accepts 206 alongside 200 and caches both. If-Range: <etag> makes the server fall back to a full 200 if the file changed upstream.
• CacheInterface gains an optional getResumeInfo so custom caches can opt in.

Scope / notes

• Node-only by design. The browser Cache API cannot store partials; OPFS/Xet is the follow-up (step 2).
• Incremental sha256 integrity verification is deliberately left for a follow-up so this PR stays focused on the resume mechanics.

Tests

New tests/utils/file_cache.test.js covers: no-partial, full 200 (no leftovers), truncated body (partial + sidecar retained, getResumeInfo correct), 206 append to completion, and 200 restart over a stale partial. Existing cache/custom_cache suites still pass; typecheck, prettier, and build are green.

Closes #1220

Opening as a draft to get maintainer direction on the Xet question in #1220 before polishing. Happy to adjust.