Add GCC 16 and Clang 22 + libc++ jobs, and cover examples in CI (#82)

[graphicsMan]

Summary:

Neither runner image ships anything newer than GCC 13, so these are the only
coverage of current compilers -- and, as it turns out, the only jobs that build
examples/ at all. Sixteen example programs, including the one the README links
as "try it live", were never compiled by CI and could rot unnoticed.

• Linux GCC 16 + libstdc++, from the Docker official image, so it is the
  compiler a user gets rather than whatever the runner ships.
• Linux Clang 22 + libc++, from LLVM's own apt repo. libc++ is deliberate:
  every other Linux job uses libstdc++, and the existing clang-trunk job already
  covers Clang against it, so this is the only configuration exercising a second
  standard library.

Both also compile an installed prefix as an external consumer -- plain -I, no
CMake target. That is what packagers ship and what broke in 1.6.1, and nothing
else in CI covers it; release.py preflight does, but only at release time,
which is the most expensive moment to find out.

Blocking rather than advisory, because an example that stops compiling should
fail CI. Both toolchains are pinned, so neither can change under us.

This replaces the advisory GCC 15.3/16.2 pair from the previous version. Those
existed to reproduce a -Wstringop-overflow report and did not: the warning comes
from Compiler Explorer's own GCC builds, not from the upstream ones, and the
same versions here are clean. It is moodycamel's, reported upstream as
cameron314/concurrentqueue#456 with a self-contained reproduction. Keeping two
jobs to chase it would have been coverage theatre.

Also adds examples/chunking_comparison_example.cpp -- the program the README
links -- so it is compiled on every run rather than living only in a Godbolt
shortlink.

Differential Revision: D117385115

[meta-codesync]

@graphicsMan has exported this pull request. If you are a Meta employee, you can view the originating Diff in D117385115.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.7%. Comparing base (528c638) to head (314b1d2).

Additional details and impacted files

@@           Coverage Diff           @@
##            main     #82     +/-   ##
=======================================
- Coverage   92.9%   92.7%   -0.3%     
=======================================
  Files         64      64             
  Lines       4990    4990             
  Branches     682     681      -1     
=======================================
- Hits        4638    4628     -10     
- Misses       352     362     +10     

see 6 files with indirect coverage changes

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[meta-codesync]

This pull request has been merged in d74898a.