test: unit-test the backtrack collaborators and relocate selector tests

[rasros]

What changed

• Add direct unit tests for the collaborators extracted in the backtrack engine split, which until now were only exercised transitively through the search suites:
  • RestartControllerTest — the Luby budget sequence, per-run budget scaling, restart-index counting, and the adaptive/unbounded schedules.
  • PhaseSavingTest — saved/target phase ordering per rephase mode, deepest-prefix capture, and the onConflictTick rotation.
  • OccurrenceIndexTest — variable→factor inversion and the LS-aliases-CP-lists invariant.
  • DeadlinePollerTest — the adaptive poll cadence, driven deterministically.
• Give DeadlinePoller a constructor-injected TimeSource.WithComparableMarks (default TimeSource.Monotonic) so its cadence is testable with TestTimeSource. No production call-site change.
• Relocate the Vsids / DomWdeg / LastConflict tests out of the 792-line BacktrackSolverTest grab-bag into selector/VsidsTest, selector/DomWdegTest, selector/LastConflictTest, matching the one-test-file-per-selector convention already used across selector/.

Why

The engine split created independently-testable units with no direct tests — a bug in the Luby sequence or a rephase-mode ordering would only surface as a search-quality wobble, impossible to localize. And the selector tests sat in the wrong file (BacktrackSolverTest rather than beside the selector under test).

Testing

New tests only; BacktrackSolverTest behavior is unchanged (tests moved verbatim). Full check lintDocs green across JVM + native + wasm.