Mixed Utility Coverage Batch

Goal

Cover the remaining high-value non-React seams after the March 24 refresh:

• core/internal/utils/mergeDeep.ts
• emoji grid and inline-index helpers
• basic-styles leaf plugin contracts
• selection non-DOM helpers
• docx-io narrow XML generator helpers

Scope

• Test-only unless a direct spec exposes a real bug.
• No /react coverage.
• No broad package sweep.

Targets

• packages/core/src/internal/utils/mergeDeep.ts
• packages/emoji/src/lib/utils/Grid/GridSection.ts
• packages/emoji/src/lib/utils/Grid/Grid.ts
• packages/emoji/src/lib/utils/IndexSearch/EmojiInlineIndexSearch.ts
• packages/emoji/src/lib/utils/EmojiLibrary/EmojiInlineLibrary.ts
• packages/basic-styles/src/lib/BaseFontBackgroundColorPlugin.ts
• packages/basic-styles/src/lib/BaseFontFamilyPlugin.ts
• packages/basic-styles/src/lib/BaseFontSizePlugin.ts
• packages/basic-styles/src/lib/BaseFontWeightPlugin.ts
• packages/basic-styles/src/lib/BaseTextIndentPlugin.ts
• packages/selection/src/lib/isSelecting.ts
• packages/selection/src/lib/extractSelectableIds.ts
• packages/selection/src/internal/utils/events.ts
• packages/docx-io/src/lib/internal/schemas/theme.ts
• packages/docx-io/src/lib/internal/schemas/styles.ts
• packages/docx-io/src/lib/internal/helpers/render-document-file.ts

Verification

1. targeted bun test on touched packages
2. pnpm install
3. filtered pnpm turbo build
4. filtered pnpm turbo typecheck
5. pnpm lint:fix