- Add external_file field to backtrace_state struct
- Add backtrace_create_state_for_file() function that marks
state for external ELF files not loaded in current process
- In backtrace_initialize, use external_file flag to:
- Pass exe=0 to elf_add for ET_DYN files (allows DWARF loading)
- Skip dl_iterate_phdr enumeration (avoids noise from current process)
This enables symbol resolution from arbitrary ELF files on disk,
with caller responsible for address translation.
