test: TCPHTTPDispatchTests purges pcapkit before every test, and test-coverage.rst records no per-test exception #2562

From the 2026-11-01 conventions audit, re-verified on 73add05a3. test-coverage.rst, "Running the Tests", bullet "Import once per class": a class that needs a fresh pcapkit calls reimport_once_per_class(self) from setUp, in place of a per-test purge_modules (#1065).

• tests/protocols/transport/test_tcp_http_dispatch_unit.py TCPHTTPDispatchTests.setUp calls purge_modules(['pcapkit']) before each test and gives no reason; its tests read TCP.__proto__ and parse fixed bytes, so no need for a fresh import is evident.
• tests/const/test_const_enum_builtin_parity.py ConstEnumRegisterFallbackTests.setUp also purges per test, but with a reason in a comment (an earlier lookup grows the registries' tables), and tests/project/test_per_class_import_guard.py (ci: periodic CI-duration review, and optimisations as the suite grows #1538) names it as the sanctioned per-test form. The page states no such exception.

Fix: switch TCPHTTPDispatchTests to reimport_once_per_class(self); add the per-test form to the bullet, with its reason and the guard's name.
Blocked: #2544 edits test_tcp_http_dispatch_unit.py; unblocks when it merges (gh pr view 2544 --json state -q .state prints MERGED).