DynamicCollection cannot write a document larger than one page: no overflow on the schema-less write path #320

DynamicCollection rejects a document larger than MaxDocumentSize (one page minus header, slot and reserve; about 16 KB with the default page size) with BLiteValidationException. A typed DocumentCollection stores such a document across overflow pages (InsertWithOverflow), and since #319 the schema-less API reads those. It still cannot write them, so:

• BLiteMigration.ToMultiFileAsync / ToSingleFileAsync refuse a database whose typed collections hold a large document (fix(core): the schema-less API reads documents a typed collection stored in overflow pages (#301) #319 makes the refusal name the collection and _id; before, the BSON reader threw).
• The LiteDB import of Moving from LiteDB: a guide, an API mapping and an import tool #297 writes to DynamicCollection; LiteDB documents can be far larger than a page.
• A server or scripting client of the schema-less API has a size limit the typed API does not have.

Suggested change

Give DynamicCollection the same overflow write path as DocumentCollection: insert and relocating update write [TotalLength][FirstOverflowPage][first chunk] into the slot with SlotFlags.HasOverflow and the rest into PageType.Overflow pages; the read side (OverflowChainReader, #319) and the delete side (DeleteSlot frees the chain, #319) already handle that layout. Candidates for sharing: move DocumentCollection.InsertWithOverflow / AllocateOverflowPage next to OverflowChainReader so both collections use one writer. Then BLiteMigration can drop its size check and MaxDocumentSize becomes the size of a single document BLite stores (today int.MaxValue-ish, bounded by the page count).

Tests: the DynamicCollection oracle (#272) with document sizes past one page; migration round trip of a typed database with a 40 KB document, both layouts.

Related: #301 (read side, fixed by #319), #297, #272.