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

TESTING.md

@@ -0,0 +1,42 @@
+# Testing
+
+The default test suite is deterministic and has no elapsed-time assertions.
+Configure a build, compile it, and run the tests with CTest:
+
+```sh
+cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
+cmake --build build
+ctest --test-dir build --output-on-failure
+```
+
+The tests independently check board dimensions, square multiplicities, bounds,
+overlap, and complete coverage.  They cover small unsatisfiable solver inputs, a
+known order-8 solution, invalid placement diagnostics, and construction of an
+order-9 solution from the order-8 fixture.
+
+## Debug and sanitizers
+
+Use a separate build directory for each configuration:
+
+```sh
+cmake -S . -B build-debug -DCMAKE_BUILD_TYPE=Debug
+cmake --build build-debug
+ctest --test-dir build-debug --output-on-failure
+
+cmake -S . -B build-asan -DCMAKE_BUILD_TYPE=RelWithDebInfo \
+  -DCMAKE_CXX_FLAGS="-fsanitize=address -fno-omit-frame-pointer" \
+  -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=address"
+cmake --build build-asan
+ctest --test-dir build-asan --output-on-failure
+
+cmake -S . -B build-ubsan -DCMAKE_BUILD_TYPE=RelWithDebInfo \
+  -DCMAKE_CXX_FLAGS="-fsanitize=undefined -fno-omit-frame-pointer" \
+  -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=undefined"
+cmake --build build-ubsan
+ctest --test-dir build-ubsan --output-on-failure
+```
+
+The sanitizer flags shown are supported by Clang and GCC.  Other compilers may
+require different flags.  Debug compilation and a feasible solver run currently
+expose the pre-existing defects tracked by issues #7 and #14 respectively; the
+test additions deliberately do not include fixes for those separate issues.