Improve performance of packet dissection and build

[polybassa]

Packet.setattr has to resolve field names before it can fall back to a plain slot assignment, and Packet.init goes through it 22 times for every layer that is dissected or built. It was the hottest function in both paths, at 18.8M calls per 21k dissected packets. Initialize the slots with object.setattr instead. Their types move to class-level annotations, since that form leaves nowhere to put an inline type comment.

PacketListField.getfield located the trailing Padding with "conf.padding_layer in p" followed by "p[conf.padding_layer]": two full recursive layer traversals per list element, ~2.7us for a two-layer element. Dissection always appends Padding as the last layer of the payload chain, so lastlayer() answers the same question in ~0.3us. This no longer descends into sub-packet fields, where a nested Padding was a false positive that truncated the list.

copy_fields_dict() and getfield_and_val() went through copy_field_value() and get_field(), spending two Python frames per field just to reach self.fieldtype[name]. Index it directly; both helpers remain for external callers.

Measured against master over 16 packet types, median of 6 interleaved rounds on a pinned core with a +/-0.6% noise floor:

dissect                              +7.7%
build (fresh packet)                +10.2%
dissect, PacketListField-heavy      +13.9%  (DNS/DHCP6/SCTP)
rebuild (dissected packet, cached)   +2.0%

No API changes. 6600 tests pass, mypy reports no new errors, and flake8 reports two fewer warnings. The 8 remaining test failures need live OpenLDAP/SMB servers and fail on master too.

AI-Assisted: yes (Claude Opus 5)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.56%. Comparing base (b1a9799) to head (ec97aa4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5085      +/-   ##
==========================================
+ Coverage   80.05%   80.56%   +0.51%     
==========================================
  Files         390      390              
  Lines       96810    96832      +22     
==========================================
+ Hits        77499    78015     +516     
+ Misses      19311    18817     -494     

Files with missing lines	Coverage Δ
scapy/fields.py	92.82% <100.00%> (ø)
scapy/packet.py	84.96% <100.00%> (+0.15%)	⬆️

... and 18 files with indirect coverage changes

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