refactor: split large JS ink modules

[markm39]

Summary

• Split ContinuousEnginePool into a pool coordinator, pooled slot component, lifecycle helpers, and shared types so page assignment and engine mounting are easier to review.
• Split NativeInkCanvas native-module discovery, public ref/prop types, and notebook bridge/export helpers into focused modules while preserving the existing public import path.
• Moved ZoomableInkViewport public types into a dedicated module and added the new source directories to the package files list so React Native source consumers still receive them.

Test plan

• npm install
• npm --prefix example install
• npm run typecheck
• npm test -- --runInBand
• npm run build
• npm run test:native:smoke
• npm run test:example:typecheck
• npm run pack:dry-run
• npm audit --audit-level=moderate
• Simplify review completed; one local clarity nit was fixed before commit.
• Security review completed for the changed code; no new bridge, payload, secret, or package exposure issues found.
• App/device verification intentionally not run per maintainer request; maintainer will run it after this PR.
• npm --prefix example audit --audit-level=moderate still reports a pre-existing Expo/PostCSS advisory (GHSA-qx2v-qp2m-jg93) that requires separate dependency work outside this refactor.