docs(planning): record horizon and cancellation decisions (#12)

docs/adr/0003-constraint-first-lexicographic-planning.md

@@ -1,3 +1,5 @@
 # Constraint-First Lexicographic Planning Without a Solver Dependency
 
+The ten-second automatic deadline is superseded by [ADR 0006](0006-user-selected-horizons-and-cancellable-calculations.md), accepted for issue #12 and awaiting implementation.
+
 VacOptimizer resolves obligation rules exactly and jointly, then applies deterministic heuristic stages for at-risk loss, ordered preferences, complete public-holiday weeks, calendar gain, work-sequence quality, supporting placements, balance expiry, and dates in that order; complete holiday weeks therefore precede broader calendar-gain and work-sequence optimization. A completed plan does not claim mathematical global optimality. The dependency-free planner targets five seconds, returns `PlanningIncomplete` if those stages are interrupted, and stops after ten seconds. A HiGHS WebAssembly spike missed that hard limit on representative inputs even on the development machine, so no solver dependency is accepted. This supersedes ADR 0001.