test: relocate LP rounding-probe tests to their own file

[rasros]

What changed

Follow-up to the LpBounding split (#953). lpRoundingProbe moved to LpPrimalHeuristics.kt, but its three tests were still in LpBoundingTest — a grab-bag mixing core-bounding and primal-heuristic tests. Relocate them into LpRoundingProbeTest, matching the LP suite's one-file-per-concern convention (ObjectiveShavingTest, FeasibilityPumpTest, LbTreeSearchTest, …).

• LpBoundingTest keeps the core: LP bounding, node pruning, root-cut harvest, frequency policy, roundUpToResidue.
• LpRoundingProbeTest holds the three probe tests (seed-and-preserve, conflict fallback, fractional relaxation); the small triangle()/sumObjective fixture the first one needs is copied over.

Why

Aligns the test files with the source split so the rounding-probe tests sit beside the code under test, and shrinks the LpBoundingTest grab-bag.

Testing

Tests moved verbatim; behavior unchanged. Full check lintDocs green across JVM + native + wasm.