tftp: extend multi-block read results in place

[KernelClint]

TFTP_read is Scapy's client automaton for downloading a file block by block. Each accepted remote
DATA block reaches immutable self.res += recvd at
scapy/layers/tftp.py:196-234,
which recopies every prior response before waiting for the next block.

From 4,000 to 16,000 full blocks, median processing grew from 82.41 ms to 1,308.05 ms, with a 2.13
exponent and an 11.5% noise floor. Patched times were 21.39 ms to 87.75 ms, with a 1.04 exponent and
a 1.7% noise floor.

This change retains the single-block bytes path, extends one bytearray after a second block, and
converts it back to bytes at the final transition. The focused regression failed on the unmodified
revision and passed with the patch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.63%. Comparing base (ba8641a) to head (5c2606f).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5110      +/-   ##
==========================================
+ Coverage   80.59%   80.63%   +0.03%     
==========================================
  Files         390      390              
  Lines       96892    96895       +3     
==========================================
+ Hits        78094    78130      +36     
+ Misses      18798    18765      -33     

Files with missing lines	Coverage Δ
scapy/layers/tftp.py	96.96% <100.00%> (ø)

... and 15 files with indirect coverage changes

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