feat: add Pivoine custom ComfyUI nodes for DiffRhythm

Add custom node wrapper PivoineDiffRhythmRun that fixes tensor dimension
mismatch error by disabling chunked VAE decoding. The original DiffRhythm
node's overlap=32 parameter conflicts with the VAE's 64-channel architecture.

Changes:
- Add comfyui/nodes/pivoine_diffrhythm.py: Custom node wrapper
- Add comfyui/nodes/__init__.py: Package initialization
- Add arty.yml setup/pivoine-nodes: Deployment script for symlink
- Update all 4 DiffRhythm workflows to use PivoineDiffRhythmRun

Technical details:
- Inherits from DiffRhythmRun to avoid upstream patching
- Forces chunked=False in diffrhythmgen() override
- Requires more VRAM (~12-16GB) but RTX 4090 has 24GB
- Category: 🌸Pivoine/Audio for easy identification

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

comfyui/workflows/text-to-music/diffrhythm-full-length-t2m-v1.json

@@ -4,7 +4,7 @@
   "nodes": [
     {
       "id": 1,
-      "type": "DiffRhythmRun",
+      "type": "PivoineDiffRhythmRun",
       "pos": [100, 100],
       "size": [400, 400],
       "flags": {},

comfyui/workflows/text-to-music/diffrhythm-random-generation-v1.json

@@ -4,7 +4,7 @@
   "nodes": [
     {
       "id": 1,
-      "type": "DiffRhythmRun",
+      "type": "PivoineDiffRhythmRun",
       "pos": [100, 100],
       "size": [400, 400],
       "flags": {},

comfyui/workflows/text-to-music/diffrhythm-reference-based-v1.json

@@ -27,7 +27,7 @@
     },
     {
       "id": 2,
-      "type": "DiffRhythmRun",
+      "type": "PivoineDiffRhythmRun",
       "pos": [500, 100],
       "size": [400, 450],
       "flags": {},

comfyui/workflows/text-to-music/diffrhythm-simple-t2m-v1.json

@@ -4,7 +4,7 @@
   "nodes": [
     {
       "id": 1,
-      "type": "DiffRhythmRun",
+      "type": "PivoineDiffRhythmRun",
       "pos": [100, 100],
       "size": [400, 400],
       "flags": {},