ABI additive-change reviews

tools/check-abi-version.sh fails a build whose installed headers changed while MESHIOPLUSPLUS_ABI_VERSION did not. This file is how that judgement gets recorded when the change really was additive.

One row per release that changed an installed header while holding the ABI version. Adding a row is a claim that every listed header change is Tier C by doc/abi.md's criterion — a new inline function, constexpr variable, type, declaration or header, or an appended enumerator — and therefore cannot affect a consumer that was already compiled. It is not a rubber stamp: the whole point of the ABI number is that a consumer may trust it instead of re-pinning the release version, so a wrong entry here ships silent memory corruption.

The releases and headers changed columns are read by the gate, not just by people.tools/check-abi-version.sh looks for a row matching the ABI version and the release version in CMakeLists.txt, then requires every header that actually changed to be named in it — paths relative to src/cpp/include/meshioplusplus/, as written below. A header you changed but did not list is a red build naming it.

That specificity is the fix for a real defect: through v9.4.0 the gate matched any row whose first column was the current ABI number, so once any ABI-3 row existed, every later header change matched it and passed — reporting "records the additive review" about a review of a different release entirely. A row for one release says nothing about the next one.

If the change edited the body of an existing inline function or template, or moved a data member, it is Tier A/B and belongs in a version bump, not in this table.

ABI	releases	headers changed	why it is additive
3	v9.3.0	formats/exodus.hpp	Adds one new inline constexpr const char* kExodusAttributePrefix. No existing declaration touched, ExodusInfo unchanged. A consumer compiled against v9.2.0 headers has an identical translation unit either way — it simply does not have the name.
3	v9.4.0	abi_version.hpp, detail/abi_version_check.hpp, mesh.hpp	All new: the ABI machinery itself. mesh.hpp gains only an #include; no type's definition changes. See the note in doc/abi.md about the one-way incompatibility this introduction creates (v9.4.0 headers reference a sentinel a ≤v9.3.0 library does not define), which fails closed at link time.