[umpireboulianne-design]

Fixes #1325

Summary

• Skip DWARF forward-declaration stubs and reset on null DIEs: Record DW_AT_declaration on DIEs, reset cur = None on Abbrev Number: 0 null DIE headers, and continue scanning in members_of() past forward-declaration or empty stub DW_TAG_structure_type entries so later compilation units with the complete runtime.g / runtime.m definitions are found.
• Ignore nested struct members: Restrict DW_TAG_member collection inside members_of() to direct children at e["depth"] == base + 1 so inner struct members at deeper nesting levels cannot overwrite outer runtime.g / runtime.m field offsets.
• Support indirect strings, hex sizes/offsets, DW_OP_plus_uconst locations, and readelf OSError: Strip .debug_str/.debug_line_str indirect/indexed string prefixes in DW_AT_name, parse hexadecimal DW_AT_byte_size and DW_AT_data_member_location values, extract DW_OP_plus_uconst offsets without misparsing N byte block prefixes, and fail closed with exit code 1 when readelf raises OSError.

Testing

• Added regression tests in tools/test_extract_offsets.py for forward-declaration/empty-stub skipping, nested struct member depth-collision protection, null DIE handling, indirect strings/hex sizes/DW_OP_plus_uconst locations, and readelf OSError handling.