When the Proxmox API's meminfo/status payload omits Available, Buffers,
and Cached (common for QEMU guest-agent and node status responses), the
code derived 'available' from Free alone — producing used = Total-Free
which counts reclaimable page cache as used memory (e.g. 94% instead of
the correct 76%).
Guest path (deriveGuestMemInfoAvailable): return 0 when cache metrics
are completely missing and only Free is available, so resolveGuestStatusMemory
tries better sources: guest-agent /proc/meminfo file-read (returns
MemAvailable), RRD memavailable, or the linked host agent.
Node path (resolveNodeMemory): always try RRD memavailable when cache
metrics are missing, not only when effectiveAvailable == 0 — previously
a non-zero Free value blocked the RRD fallback.
Refs #1501
Tighten v5-to-v6 upgrade safety, release installability, provider MSP mode handling, AI cost accounting, metrics flushing, and frontend guardrails for the v6.0.0 GA candidate.
