Fix a double free issue when start == goal

[justinhj]

Summary

Fixes a bug where initiating and completing a search where the start node is already the goal (`start == goal`) causes

a use-after-free and double-free of m_Start.

### Root Cause
1. When `start == goal`, `SearchStep()` succeeds on the first step. Because `n->m_UserState.IsSameState(m_Start-

m_UserState)is true, the backward solution-chaining loop is skipped andm_Start->childremainsnullptr. 2. FreeUnusedNodes()immediately iterates overm_NodeMapand deletes any node where!n->child. Since m_Start-
child == nullptr, m_Startwas erroneously freed as an "unused" node. 3. Subsequent calls toGetSolutionStart()resulted in a use-after-free. 4. WhenFreeSolutionNodes()was called, it executed itselsebranch (specifically designed to clean upm_Start andm_Goalwhenm_Start->child == nullptr), freeing m_Start` a second time.

### Impact
- In debug builds, `m_AllocateNodeCount` drops to `-1`, triggering an assertion failure in `EnsureMemoryFreed()`.
- With `FixedSizeAllocator`, freeing the same block twice causes a self-referential cycle in the intrusive free-list

(pNode->pNext = pNode), silently corrupting the allocator and causing subsequent allocations to alias the same memory.
Because FixedSizeAllocator manages memory within a single monolithic buffer, AddressSanitizer (ASan) does not detect
this corruption in release builds.

### Fix
- In `FreeUnusedNodes()`, skip `m_Start` (`if (n != m_Start && !n->child)`), ensuring lifetime ownership of `m_Start`

remains solely with FreeSolutionNodes().

### Testing
- Added unit test `Start Node Equals Goal Node` in `tests.cpp` verifying successful search completion, solution

traversal, and memory cleanup without assertions or leaks.
- Verified all existing tests and example executables (findpath, 8puzzle, minpathbucharest, and bench) pass.