fix(PkgBuilder): size the PlayGo chunk hash entry to a fixed point

[skippdot]

Fixes #26 (also reported as maxton#88, maxton#64, maxton#56).

BuildPkg pre-allocates the PlayGo chunk hash entry (4 bytes per 64 KiB of the final package) from a one-shot estimate. The estimate misses two things:

• the entry itself (and the Metas entry, laid out as Entries.Count * 32) can push the body over the next 0x80000 boundary;
• that boundary adds more chunks.

For some package sizes this throws Playgo Chunk hash file was not allocated enough space.

Fix: compute the size to a fixed point, mirroring the actual layout. The exact size is still set later, as before.

Test: ChunkShaTests.ChunkShaAllocation_HoldsForAnyPfsSize sweeps ~28k PFS sizes from 1 to 70 GB through BuildPkg without writing anything. Master fails for several sizes; it passes with the fix. Marked Slow (~75 s).