Add a Mumbling bitmap reader for v4 manifest deletion vectors #1125

Iceberg v4 stores manifest deletion vectors inline on root manifest entries (manifest_info.dv, tracking.deleted_positions and tracking.replaced_positions). They are encoded in the new Mumbling bitmap format, whose spec is still a draft (apache/iceberg#16518). Iceberg Java has a reader (apache/iceberg#16747), but iceberg-cpp has none, and no C/C++ library implements the format. Planning scans of v4 tables needs one (part of #1107).

Proposal: add an internal, read-only MumblingBitmapView. It wraps the serialized bytes without copying them, validates everything that memory safety depends on, and supports Contains() and Cardinality(), the same surface as Java's ManifestBitmap. While the spec is a draft it stays in an *_internal.h header that isn't installed. Writing bitmaps and applying them in the v4 manifest reader would be follow-ups.