Unified GTT memory detection for AMD Strix Halo (gfx1105) and Strix Point (gfx1102) APUs.

Scope:

• Detect unified-memory GPUs via KFD gfx_target_version (gfx1102, gfx1105)
• Compute (vram_used + gtt_used) / (vram_total + gtt_total) on unified GPUs
• Skip nvidia-smi subprocess spawn entirely on AMD systems
• Add unit tests for has_unified_memory() and vram ratio calculation
• Refactor: replace goto with structured control flow
• Future-proof: extend to new gfx families beyond gfx1102/gfx1105

Branch: fix/amd-unified-vram
Severity: Log flood (~600 lines/min), false eviction triggers