themes: only enqueue MMCE decorator art on selection transitions

[NathanNeurotic]

Motivation

• Prevent draw-loop driven MMCE cover request side-effects by separating request initiation from display-time retrieval so IO enqueueing only happens on real selection transitions.
• Keep existing behavior for non-MMCE modes (USB/HDD/SMB/MX4SIO/Apps/iLink) and avoid changing art naming/path logic.

Description

• Add lightweight per-items-list selection state to items_list_t in include/themes.h as lastSelectedItemId and lastSelectedStartup[256] to detect selection transitions.
• In src/themes.c drawItemsList compute mmceSelectionChanged when list->mode == MMCE_MODE and the decorator has a cache by comparing item->item.id and the startup string from list->itemGetStartup(list, item->item.id).
• For MMCE mode only, call the enqueue-capable path (getGameImageTexture / cacheGetTexture) only when the selection actually changed for the selected row, and otherwise use cacheGetTextureIfReady for display-only retrieval to avoid queuing IO from the draw path.
• Initialize the new tracking fields in initItemsList so lastSelectedItemId starts as -1 and lastSelectedStartup is empty, and leave non-MMCE retrieval logic unchanged.

Testing

• Ran make -j2 (project build) which failed in this environment because required language YAML files are missing (lng_src/English.yml, lng_src/Albanian.yml), so a full build verification could not complete.
• Attempted to build the object with make obj/themes.o which failed due to no matching Makefile rule in this environment, so further compile validation was not possible here.

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Codex Task

[gemini-code-assist]

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