test: add independent placement validation

Establish a CTest harness before changing solver behavior. Validate board dimensions, multiplicities, bounds, overlap and coverage against independent placement data, including known order-8 and constructed order-9 fixtures.

Document Debug and sanitizer workflows while keeping the defects tracked by #7 and #14 separate. Prepare a result adapter so the feasible solver regression can be enabled with the completion fix.

Tests: Release, ASan and UBSan CTest suites (3 passed each)

Refs: #1

main.cc

@@ -67,6 +67,9 @@ namespace {
 
     [[nodiscard]] auto length() const noexcept -> size_t { return length_; }
 
+    /** Get the square placements in this result. */
+    [[nodiscard]] auto squares() const noexcept -> std::vector<Square> const & { return squares_; }
+
     /** Output the grid. */
     auto output() const -> void {
       std::string out(length_ * length_, '.');
@@ -303,6 +306,7 @@ namespace {
   }
 } // anon namespace
 
+#ifndef PARTRIDGE_TESTING
 int main(int argc, char **argv) {
   auto n = (argc == 1) ? 8 : std::atol(argv[1]);
   auto const grid = find_solution(n);
@@ -310,3 +314,4 @@ int main(int argc, char **argv) {
   grid.output();
   return 0;
 }
+#endif