Replace linear page_table_ scans in AllocRange with a std::map-based
free block index that tracks contiguous free regions.
Insertions now coalesce with adjacent blocks on release and AllocFixed
uses the targeted tracker for pure reserves and falls back to a full rebuild
for mixed-state commits.
Also fixing PhysicalHeap leaking parent memory on child allocation failure,
Reset() not restoring unreserved_page_count_ and some incorrect method names
in PhysicalHeap error messages
