Fix rendering issues in Chapters 9 and 12 and document dependency gaps #1178

While reproducing the book on Ubuntu 26.04, I encountered and fixed several issues:

1. Chapter 9 (09-mapping.Rmd): Fixed an 'external pointer is not valid' error for the 'nz_elev' object. This occurs when the raster object is carried over between sessions but its C++ pointer becomes null. Added a 'tryCatch' to reload the raster if the pointer is invalid.
2. Chapter 12 (12-spatial-cv.Rmd): Fixed a syntax error where 'data.table' style indexing was used on a standard 'data.frame', causing a 'column not found' error.
3. Dependency Gaps: Noticed that 'geocompkg' is missing several dependencies in its 'Suggests' field that are required for a full build (e.g., 'z22' was missing from 'Remotes' and several others required manual installation).

Local fixes have been applied to the Rmd files.