QL: preserve full-step endpoints after mode-3 refinement

[krystophny]

Closes #123. This child is stacked on #125, #119, #121, and #124 and remains open against main.

Summary

• keep both mode-3 field-line endpoints on full Runge-Kutta points
• when the requested subdivision count is odd, double the complete subdivision vector before constructing the refined grid
• test endpoint parity, original-node retention, ordering, and field-line reversal equivariance through the production divider

Invariants

The repair leaves the second-order source stencils and their 5/12, 2/3, and -1/12 weights unchanged. It preserves all requested points, local refinement ratios, coordinate orientation, trapped/passing mapping inputs, source signs, Fourier convention, CGS units, field-line boundary conditions, ABI, and storage layout. Even-count grids are bitwise unchanged. An odd-count retry grid uses twice the requested intervals, so both endpoints are full points and every istep +/- 2 access remains inside the solve range.

The added resolution is limited to mode-3 retries whose requested interval count is odd. No branch in this stack is authorized to merge before the programme-wide gate.

Verification

Test fails before fix

With the production divider unchanged:

$ ctest --test-dir build -R '^phi_divider_parity_test$' --output-on-failure
0% tests passed, 1 tests failed out of 1
FAIL: refinement made the final Runge-Kutta point a half step

The production mapping for [2,1,2,2] ended at index 7. Original-node retention, ordering, and reversal checks already passed, isolating the endpoint contract.

Test passes after fix

$ fo
Static: OK (331 modules)
Build: OK
Tests: OK
Lint: OK
All stages passed

$ make test
100% tests passed, 0 tests failed out of 10

$ ctest --test-dir build -R '^phi_divider_parity_test$' --output-on-failure
100% tests passed, 0 tests failed out of 1

$ git diff --check
<no output>

The odd request maps to [4,2,4,4], ending at full-step index 14. The reversed request produces the reflected mapping, and every original point remains ordered and represented.

The 16 separately managed external-data regressions remain registered under regression;external-data; this branch does not mark that unavailable-data gate passed. A bounds-checked HELCORE production run and golden-record comparison remain integration gates.