## Problem
`getSpineIndexForTocIndex()` and `getTocIndexForSpineIndex()` access
`toc[tocIndex]` and `spine[spineIndex]` without validating indices are
within bounds. Malformed EPUBs or edge cases could trigger out-of-bounds
access.
## Fix
Added bounds validation at the start of both functions before accessing
the arrays.
## Testing
- Builds successfully with `pio run`
- Affects: `lib/Epub/Epub.cpp`
