Over-exposed store internals: 3 stores with unused public exports

## Recommendation

### ♻️ REFACTOR: Make internal store helpers private

Three stores export internal helpers that are never consumed externally.

## What Was Found

### modelToNodeStore.ts — 3 Unused Public Exports
- `registerNodeProvider`, `quickRegister`, `registerDefaults` — only used internally
- Public API (`getRegisteredNodeTypes`, `getCategoryForNodeType`, etc.) is actively used by 10+ consumers

### maskEditorDataStore.ts — 3 Unused Exported Getters
- `loadError`, `hasValidInput`, `hasValidOutput` — zero external consumers

### templateRankingStore.ts — 1 Unused Export
- `computeFreshness` — only used internally by `computeDefaultScore` and `computePopularScore`

## Context

Reducing the public surface area of these stores improves encapsulation and makes the API clearer for consumers.

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**Category:** Wiring Gap | **Confidence:** HIGH | **Tools:** Semantic analysis
_Filed by audit-code skill_

Part of #11022

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11050-Over-exposed-store-internals-3-stores-with-unused-public-exports-33e6d73d365081ffb715e12543007cb3) by [Unito](https://www.unito.io)