docs(documents): mark clipBucketsToRange as @internal (#385)

The function has a single in-source call site (TimelineDensityFilter)
but is exported so timeline.spec.ts can pin its boundary semantics
without rendering the orchestrator. Note that explicitly so future
readers don't treat the export as a public API contract.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/document/timeline.ts

@@ -61,6 +61,9 @@ export function fillDensityGaps(
  * `[fromInclusive, toInclusive]` ISO date range. When either bound is null the
  * input array is returned unchanged. Used by the timeline's zoom-in tool to
  * narrow the visible bars without refetching data.
+ *
+ * @internal Sole call site is `TimelineDensityFilter.svelte`. Exported so the
+ * unit suite (`timeline.spec.ts`) can pin the boundary semantics directly.
  */
 export function clipBucketsToRange(
 	buckets: MonthBucket[],