Add dedicated unit tests for BlockRow.svelte

[brylie]

#239 extracted src/lib/components/BlockRow.svelte as a new, independently reusable component — the shared move handle, block action menu, multi-select/just-navigated highlighting, to_do/bulleted/numbered/toggle gutter, and divider/quote/freeform-text content dispatch, consumed by both +page.svelte's top-level block loop and ColumnsBlock.svelte's per-column loop.

Every other reusable component under src/lib/components/ has its own dedicated *.svelte.test.ts (BlockActionMenu, BlockEditor, CalloutBlock, ChildPagesBlock, CollectionViewBlock, ColumnsBlock, SyncedBlockUsage, and more) — BlockRow.svelte does not. Its behavior today is exercised only indirectly, through ColumnsBlock.svelte.test.ts and page.svelte.test.ts.

A regression introduced directly inside BlockRow.svelte — in the customContent/customBlockTypes extension-point gating, the heldByActorLabel placeholder, the numberedListIndex computation, or an optional prop like onToggleCollapse/onLinkShortcut/onArrowUpAtStart/onArrowDownAtEnd — would only surface as a confusing failure in one of those two callers' suites rather than pointing at BlockRow directly. #227 (toggle blocks becoming real containers) is already expected to become a third consumer of BlockRow, and will have nothing but those two indirect suites to build confidence on its own contract.

• src/lib/components/BlockRow.svelte.test.ts renders BlockRow directly (not through a caller) and covers: the move handle + block action menu wiring, selection/just-navigated highlight classes, the to_do/bulleted_list_item/numbered_list_item/toggle gutter (onToggleCollapse both provided and omitted), the divider/quote/default-text content dispatch, the heldByActorLabel placeholder, and the customContent/customBlockTypes/trailingContent/insideContent extension points both present and absent.

Done when: BlockRow.svelte.test.ts exists and passes, exercising BlockRow in isolation across the cases above, independent of ColumnsBlock.svelte.test.ts and page.svelte.test.ts.

[brylie]

Set P2 (unset → P2): a test-coverage gap on a component #239 just extracted, in the same spirit as the sibling consolidation issues from that release (#229, #241). Not blocking anything today, but worth landing before or alongside #227 (toggle nesting), which is expected to become BlockRow's third consumer and would otherwise build on an untested contract.