iris-gui: move the jitv2 persistent code cache toggle to the General tab

[iblowmymind]

Summary

• PR jitv2: move the persistent code cache toggle into [jitv2] config #162 put the jitv2 persistent code cache checkbox (cfg.jitv2.cache/cache_dir) on the Debug tab. Tab::visible hides the Debug tab entirely in lightning builds (build_features::LIGHTNING) — but jitv2's persistent cache is an independent cargo feature from lightning, so a lightning,jitv2 build (e.g. the "Premiere" profile with jitv2 added) had no UI path to the toggle at all.
• Moves the checkbox + cache-dir field into the General tab (always visible), right after the Processor/CPU section, still gated on build_features::JITV2 so it only appears in jitv2 builds.
• Dropped the Debug tab's "not built — rebuild with..." fallback label when relocating — the General tab doesn't otherwise narrate build features inline (e.g. the Ultra64 toggle there just doesn't apply if irrelevant), so the section is now silently omitted rather than explained, consistent with the rest of that tab.

Test plan

• cargo check -p iris-gui --features iris/jitv2
• cargo check -p iris-gui (default, no jitv2)
• cargo check / cargo check --features jitv2 (CLI crate, unaffected but re-verified)
• Manual click-through of the relocated checkbox (not done this round)