Validate PayloadAlignment when loading CAS index headersb/fixups

CasContainerStrategy::ReadIndexFile previously accepted any nonzero
PayloadAlignment from the on-disk index header. The value is later
used as the multiplier in BlockStoreDiskLocation::GetOffset
((offset_bits_28) * PayloadAlignment) to translate packed disk
locations into block-file byte offsets.

A corrupt or malformed header with a non-power-of-two or outlandishly
large PayloadAlignment would silently skew every offset computed from
the loaded index, causing reads at wrong positions in block files.
The magic/version/checksum gate most corruption, but this is
defense-in-depth for bit-rot that happens to preserve the checksum,
and matches the power-of-two invariant that is already asserted when
the alignment is set at runtime (line 171).

Require IsPow2(PayloadAlignment) and cap at 4096 (sector-sized),
which covers the 16-byte default and leaves room for future tuning.

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