[runtime/blob] partial page indicator subject to undetectable corruption

[roberto-bayardo]

Related: #3837

Since the length field of the CRC slot isn't part of the CRC, if corrupted it would not be detectable. This could lead to us loading a page without error and treating it as the wrong size (e.g. partial even when it is not partial).

[roberto-bayardo]

Note that we already accept some cases of undetectable corruption, such as shortening a blob (any trailing partial page is trimmed).