[SizzleUnrlsd]

Summary

• Add a Coverage workflow on Linux. It builds with --coverage -fprofile-update=atomic, runs the whole suite, then runs gcovr 8.6 on src/ and include/, and publishes a markdown summary to the job summary.
• The job fails when line coverage drops below 88%, the value measured on this branch (--fail-under-line). The summary is still published when it fails. A comment next to the threshold records where it comes from, so it can be raised as coverage grows.
• Atomic counters are required: with the default counters, the multithreaded tests corrupt them and gcovr aborts on negative hit counts (GCC bug 68080).
• gcovr only searches the library, example and test objects, because CMake's compiler detection also leaves coverage data under build/CMakeFiles that gcov cannot resolve.

With the tests from #14 and #15 (and #17, #18), line coverage goes from 48.7% on main (llvm-cov) to 88.7% with GCC. What remains uncovered is mostly the defensive branches: EINTR and partial-write retries, the gmtime failure path, and the unreachable if (!started) branch in write_hex().

Stacked on #43.

Verification

• CI on this branch (GCC 13, gcovr 8.6): lines 88.7% (525/592), functions 96.6% (57/59), branches 63.3%. The first run exposed the negative-hit problem, and the second one the provisional 90% threshold.
• Locally (gcovr with llvm-cov gcov, AppleClang 17): lines 91.7%. --fail-under-line 90 exits with 0 and --fail-under-line 99 exits with 2.

Closes #19