Fixed various findings about dyn_array to harden code.
- Harden dyn_array overflow handling, clarify ownership, and make access helpers safer
- Preserve const in access macros
- Allow empty-array range helpers after free
- Document destroy null no-op: dyn_array_destroy(struct dyn_array **array_p)
- Refine seek offset range guard
- Fix portable off_t overflow check
- Fix overflow and portability review findings
- misc code and comment cleanup
Also fixed issue that tripped up CodeQL parallel make.