docs: remove bind-propagation=shared, document runc v1.3.5 crash

runc v1.3.5 (Docker 29.x / Unraid 7.3+) crashes on bind-propagation=shared
with any container — not GPU-specific. Ramdisk tmpfs is already MS_SHARED
at kernel level; propagation is automatic and the option is unnecessary.

- Remove bind-propagation=shared from all example Extra Parameters
- Update "Why shared Is Required" → "Mount Propagation — Unraid 7.3+"
- Update GPU troubleshooting section to cover the general runc crash
- Verify commands updated (no longer checking for Propagation: shared)
- Historical notes preserved explaining why it existed before

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README-User_Script_Plug-in.md

@@ -155,10 +155,11 @@ auto-flip to SSD, missing transcoding-temp → recreate immediately.
 
 **Output when healthy:** session list only — no flip messages = ramdisk managing fine
 
-> ⚠️ The Emby Docker container must have
-> `--mount type=bind,source=/mnt/ram-transcode,target=/ext-ram-transcode,bind-propagation=shared`
-> in Extra Parameters. Without `shared` propagation, symlink flips are silently
-> ignored after the first flip and all sessions land permanently on SSD.
+> ⚠️ The Emby/Jellyfin Docker container must have
+> `--mount type=bind,source=/mnt/ram-transcode,target=/ext-ram-transcode`
+> in Extra Parameters. Do **not** add `bind-propagation=shared` — runc v1.3.5+
+> (Docker 29.x, Unraid 7.3+) rejects it with a mountinfo error. The ramdisk tmpfs
+> is already MS\_SHARED at the kernel level — propagation is inherited automatically.
 
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