fix(lang-v2): cap flattened Accounts HEADER_SIZE at 255

[akash-osec]

Fixes finding AI # 32

Nested Accounts could flatten past 255 slots while only the top-level field count was capped, leaving mute-mask/offset math out of domain. So the fix applies here and now compile-time assert that flattened HEADER_SIZE fits the 256-bit duplicate-tracking domain.

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Additional details and impacted files

@@              Coverage Diff               @@
##             anchor-next    #4909   +/-   ##
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  Coverage               ?   65.03%           
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  Files                  ?      127           
  Lines                  ?    15877           
  Branches               ?        0           
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  Hits                   ?    10325           
  Misses                 ?     5552           
  Partials               ?        0           

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