ref(🥞): deprecate theme.zIndex scale

[natemoo-re]

Summary

• Add @deprecated JSDoc to theme.zIndex object and all properties (except sentryErrorEmbed)
• No behavioral change — purely documentation to guide developers toward the Layer primitive

🥞 Layer Primitive Series

• ref(🥞): add Layer primitive with portal outlets and stacking context isolation #114516 nm/zindex/layer-primitive — Layer component + hooks
• ref(🥞): restructure DOM order for paint-order stacking #114520 nm/zindex/dom-order — DOM restructuring for paint-order stacking
• ref(🥞): wire portal consumers to Layer outlets #114549 nm/zindex/wire-portals — Wire portal consumers to Layer outlets
• ref(🥞): remove structural z-index references (drawer, modal, toast, sidebar) #115959 nm/zindex/remove-structural-zindex — Remove structural z-index refs
• ref(🥞): remove portal z-index references (dropdown, tooltip, hovercard) #115957 nm/zindex/remove-portal-zindex — Remove portal z-index refs
• ref(🥞): replace local z-index theme refs with bare z-index: 1 #115956 nm/zindex/remove-local-zindex — Replace local z-index refs with bare z-index: 1
• ref(🥞): deprecate theme.zIndex scale #115961 nm/zindex/deprecate-zindex — Deprecate theme.zIndex scale ← this PR
• ref(🥞): add no-z-index lint rule banning z-index declarations #115963 nm/zindex/lint-ban — Lint rule banning z-index/zIndex
• ref(🥞): remove theme.zIndex and remaining z-index infrastructure #115964 nm/zindex/final-cleanup — Remove theme.zIndex entirely

Test plan

• No behavioral change — verify pnpm run typecheck passes

[github-actions]

📊 Type Coverage Diff

✅ No new type safety issues introduced. Coverage: 93.59%