QL: retain propagator join solver diagnostics

[krystophny]

Refs #127. This child is stacked on #126, #125, #119, #121, and #124 and remains open against main.

Summary

• report the exact DGBSV status and solve stage when intermediate propagator joining fails
• report old/new propagator tags, matrix dimensions, and finite matrix/RHS counts
• cover the production diagnostic with a real singular DGBSV solve

Invariants

Successful joins are unchanged. On failure, the existing stage error code and stop path remain unchanged; the change only retains diagnostics already available at the call site. It does not change the join matrix, right-hand side, pivoting, solver acceptance, discretization, convergence criteria, source weights, field-line boundary conditions, ABI, array ownership, or storage layout.

No branch in this stack is authorized to merge before the programme-wide gate.

Verification

Test fails before implementation

With only the behavioral test applied at parent 4b00b88:

$ cmake --build build --target test_join_failure_diagnostic
Fatal Error: Cannot open module file 'join_diagnostics_mod.mod'
ninja: build stopped: subcommand failed.

The parent has no production diagnostic API that can retain the LAPACK status.

Test passes after implementation

$ fo
Static: OK (333 modules)
Build: OK
Tests: OK
Lint: OK
All stages passed

$ make test
100% tests passed, 0 tests failed out of 11

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

$ git diff --check
<no output>

The singular 2x2 fixture returns info=2; the production diagnostic prints stage 2, status 2, both tag pairs, dimensions 2 2, and finite counts 4/4 and 2/2.

The 16 separately managed external-data regressions remain registered under regression;external-data; this branch does not mark that unavailable-data gate passed. Root-cause repair for #127 remains a separate child after a replayable join fixture identifies the failing pivot/input state.