Split protocol_test.cc into four translation units

[philipcraig]

Part of #423.

Splits protocol_test.cc by topic so that a change to one topic recompiles 11 to 21 s of tests rather than 50 s. This is for incremental rebuilds; it is not expected to change CI times, and the CI figures below show none. The tests themselves are unchanged.

Per-TU compile time, -O2 -c, GCC trunk 20260906 on a 2-core Codespaces VM:

TU	Time
protocol_test.cc (before)	50.5 s
protocol_test.cc	15.9 s
protocol_view_tests.cc	20.6 s
protocol_member_function_tests.cc	17.5 s
conformance_tests.cc	11.3 s

A full build does about 15 s more work, because each new TU pays the 4.8 s gtest and protocol.hh baseline again. That is why CI does not get faster: the shorter critical path only helps on a machine with more cores than TUs.

CI, one run each, main (f01e297, scheduled) against this PR. The CMake figures are the ninja build span and, in brackets, the time until the last test object finished, both from log timestamps; these jobs have no build cache. The Bazel and sanitizer jobs restore a disk cache, so they are not compared. The differences are within run-to-run variation.

Job	main	this PR
CMake, GCC trunk Release	73 s (61 s)	88 s (58 s)
CMake, GCC trunk Debug	51 s (51 s)	50 s (50 s)
CMake, GCC-16 Release	57 s (48 s)	56 s (36 s)
CMake, GCC-16 Debug	28 s (27 s)	36 s (35 s)
CMake, GCC-16 macOS Release	35 s (30 s)	39 s (33 s)
Runtime coverage, build and test step	86 s	92 s
Consteval coverage, probe step	205 s	283 s

The consteval coverage script names its TUs, so its list gains conformance_tests.cc and protocol_view_tests.cc; protocol_member_function_tests.cc reaches no probe point the others miss and is left out. Coverage is unchanged at 194 of 216 probe points. Run back to back on the 2-core VM the probe takes 7m26 on main and 6m52 here; the CI probe step is slower in this one run, where #425, which has main's TU list, took 243 s.

#421 and #356 add tests to protocol_test.cc in regions that move, so whichever lands second needs its new tests re-homed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.21%. Comparing base (6b86c62) to head (b2c0f1e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #424   +/-   ##
=======================================
  Coverage   81.21%   81.21%           
=======================================
  Files          16       16           
  Lines        1299     1299           
  Branches      214      214           
=======================================
  Hits         1055     1055           
  Misses         45       45           
  Partials      199      199           

Flag	Coverage Δ
consteval	90.04% <ø> (ø)
runtime	79.30% <ø> (ø)

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