docs(design): note graceful degradation of getter read paths

State the cross-adapter contract that a getter returns a drive or volume it
cannot fully resolve (missing by-id link, failed JBOD device node, degraded
array) with empty paths and its status, rather than failing and dropping the
whole controller's inventory. Documents the behavior the megaraid, ssacli and
storcli2 fixes now guarantee.

Issue: ARTESCA-17960

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: g-carre

DESIGN.md

@@ -127,6 +127,16 @@ flags. This is not for idempotency but to minimize real mutations and to skip
 fields the (lossy) getter reports as `Unknown` when the caller did not change
 them — avoiding a spurious "unsettable" rejection on an untouched field.
 
+Getter read paths degrade gracefully. When a drive or volume in a multi-member
+array cannot be fully resolved — e.g. a degraded-but-online array whose udev
+`by-id` link is missing, or a failed JBOD drive whose device node is gone — the
+getter returns that entity with its status and empty `DevicePath`/`PermanentPath`
+rather than failing, so one unhealthy drive does not drop a controller's whole
+inventory. (The megaraid create/settle path is deliberately stricter: a
+just-created volume whose device node has not yet appeared is treated as
+not-ready and retried after a bus rescan, so there a failed path resolution is
+still an error.)
+
 ### Adapters
 
 #### MegaRAID / PERC (storcli, perccli)