Add CI test checking header only edition does not collide symbols

[yaroslavaristov]

Closes #10

Problem

programs/collision-check already existed but didn't actually test for
a collision. It built a single shared library (lib.cpp, compiled
with LIBNO=1) and ran nm -D on it to check for non-inline exported
LLFIO symbols. LIBNO was never used with a second value anywhere in
the project, so there was no second library to collide with, and
nothing ever linked two independent header-only inclusions of LLFIO
together to see what happens.

What this PR does

Builds two independent shared libraries, each of which includes the
LLFIO headers in header-only mode in complete isolation from the
other, then links both into a single executable. If header-only LLFIO
ever leaks a non-inline symbol, this either fails outright at link
time (duplicate symbol) or produces a silent ODR violation at load
time — both of which now show up as a CI failure instead of going
unnoticed.

Changes

• programs/collision-check/lib.cpp → programs/collision-check/lib1.cpp
  Renamed for symmetry with the new lib2.cpp. Content unchanged.

• programs/collision-check/lib2.cpp (new)
  Second translation unit, identical to lib1.cpp, independently
  including llfio.hpp and exporting make_file2(). Built into its
  own shared object with LIBNO=2.

• programs/collision-check/main.cpp (new)
  Links both shared objects into one executable, calls make_file1()
  and make_file2(). This is the actual collision test: if the two
  header-only inclusions clash, this target fails to link or crashes
  at load time.

• programs/collision-check/CMakeLists.txt (rewritten)

  • Builds llfio-collision-check-lib1 and llfio-collision-check-lib2
    from lib1.cpp / lib2.cpp in a loop.
  • Builds llfio-collision-check executable from main.cpp, linked
    against both libraries.
  • Registers the executable with add_test(NAME llfio-collision-check ...)
    so it runs under ctest.
  • Keeps the existing nm -D | grep non-inline-symbol check, now run
    against both shared objects instead of just one.

• programs/CMakeLists.txt
  Added enable_testing() right after the project() call. Without
  it, add_test() in the collision-check subdirectory never registers
  with ctest, so the new test would silently not run.

• .github/workflows/programs.yml
  Added a Run collision-check step after Build, restricted to
  Linux, that runs ctest -R llfio-collision-check --output-on-failure.
  Previously the workflow only compiled programs/; it never executed
  anything, so this test would build but never actually run in CI.

Notes

• Still Linux-only, same restriction as the original collision-check
  (the WIN32 OR APPLE guard at the top of the CMakeLists is
  unchanged) — the nm-based symbol visibility check is POSIX/ELF
  specific.
• No changes to any LLFIO library code — this only adds test
  infrastructure.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.