Versioning and science freeze

xstar-tools tracks several independent identities:

• package version — product/API/docs/packaging evolution (0.6.90.5.8 here);

• science revision — accepted Python scientific behavior (0.6.90.5.5);

• frozen C++ science baseline — accepted science-changing native milestone (0.6.90.5.5);

• C API ABI — 60487;

• production-zone ABI — 6048110.

A documentation, CLI, API, or packaging release must not change the science revision just because its distribution version changes.

PARITY_FREEZE.md and qualification/parity_freeze.json define the frozen scientific invariants, accepted comparator policy, and structural exceptions. Any intentional scientific reopening requires explicit qualification and a new science revision.

The 0.6.90.5.5 science revision was explicitly requalified and re-frozen in package 0.6.90.5.8 using an all-elements mnabund=1 FORTRAN-vs-C++ host comparison. Package 0.6.90.5.6 and 0.6.90.5.7 changed terminal diagnostics only and therefore do not advance the science revision.