refactor: finish package split cleanup — remove ios renderer shims and deduplicate createVoltraComponent

[burczu]

Summary

Follow-up to #149. The package split was done mostly automatically, leaving behind a few cases of code duplication. This PR finishes the cleanup.

• Delete packages/ios/src/renderer/ shims — 7 one-liner re-export files (context-registry, dispatcher, element-registry, flatten-styles, render-cache, stylesheet-registry, types) were left over from the split, making @use-voltra/ios look like it had its own renderer. Removed them and cleaned up the barrel index.ts.
• Re-export createVoltraComponent from core — the implementation was duplicated verbatim in @use-voltra/ios and @use-voltra/android. Core already owns and exports it — both platform packages now re-export from there. Internal consumers within each package continue to import from the local path, transparently resolving through the re-export.

What was considered but left alone

• chart-types.ts (ChartDataPoint, SectorDataPoint) — identical 2-line type file in both platform packages. Moving to core would give a platform-agnostic renderer package chart-domain knowledge. Duplication is acceptable at this size.
• useUpdateOnHMR.ts — identical 20-line RN hook in both client packages. Uses __DEV__ and Metro's global.__accept, so it can't go to core. No appropriate shared home without a new package for 20 lines.
• widgets/server-credentials.ts — near-identical in @use-voltra/ios-client and @use-voltra/android-client, differing only in the native module import, error message platform label, and caller. The differences are genuine platform facts; there is no valid shared home (clients cannot depend on each other, and both are restricted from importing @use-voltra/core directly).
• VoltraRNNativeComponent.ts — present in both client native/ dirs, differing only in the registered native view name ('VoltraView' vs 'AndroidVoltraView'). Intentional — different native components for each platform.
• fonts.ts across expo-plugins — not actually duplicated. packages/expo-plugin/src/utils/fonts.ts is a shared utility; the platform-specific plugin files (ios-client/expo-plugin and android-client/expo-plugin) import resolveFontPaths from it. Structure is correct.
• Lint rules for RN/expo in @use-voltra/ios and @use-voltra/android — already in place in refactor: migrate native module from Expo Modules API to React Native Turbo Modules #149.
• Dependency audit — all packages clean; RN and expo only appear in peerDependencies.

Test plan

• tsc --noEmit passes on @use-voltra/ios and @use-voltra/android
• @use-voltra/ios test suite passes (5/5)
• @use-voltra/android test suite passes (6/6)
• oxlint passes on all changed packages