fix(nano): incorrect lower bound in _SizedIntNCType

[jansegre]

Motivation

Lower bound for signed integer was incorrectly using byte-size instead of bit-size.

Acceptance Criteria

• Fix lower bound calculation of sized signed integer serializer.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[github-actions]

Bencher Report

Branch	fix/sized-int-lower-bound
Testbed	ubuntu-22.04

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Benchmark	Latency	Benchmark Result minutes (m) (Result Δ%)	Lower Boundary minutes (m) (Limit %)	Upper Boundary minutes (m) (Limit %)
sync-v2 (up to 20000 blocks)	📈 view plot 🚷 view threshold	1.74 m (+6.15%) Baseline: 1.64 m	1.48 m (84.79%)	1.80 m (96.50%)

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[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.57%. Comparing base (4daccef) to head (23eeef7).

Files with missing lines	Patch %	Lines
hathor/nanocontracts/nc_types/sized_int_nc_type.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1317      +/-   ##
==========================================
- Coverage   85.59%   85.57%   -0.02%     
==========================================
  Files         424      424              
  Lines       31986    31989       +3     
  Branches     4967     4967              
==========================================
- Hits        27377    27376       -1     
- Misses       3607     3610       +3     
- Partials     1002     1003       +1     

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The merge-base changed after approval.