feat: enhance documentation for leave planning concepts and solver recommendations

docs/adr/0001-ordered-heuristic-optimization.md

@@ -1,3 +1,5 @@
 # Ordered Heuristic Optimization
 
+Superseded by [ADR 0003](0003-constraint-first-lexicographic-planning.md).
+
 VacOptimizer applies enabled planning rules in user-defined order and then uses a deterministic heuristic to choose placements; it does not backtrack or promise a globally optimal plan. This keeps results predictable and fast for the personal planner while leaving room to improve the heuristic later.

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

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+# Constraint-First Lexicographic Planning Without a Solver Dependency
+
+VacOptimizer resolves obligation rules exactly and jointly, then applies deterministic heuristic stages for at-risk loss, ordered preferences, calendar gain, work-sequence quality, supporting placements, balance expiry, and dates in that order; 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.