Ported ReRAC's (vendor/rerac/crates/rc-formats/src/level_overlay.rs) mask /
Relocation machinery into gc-formats:
- parse_overlay_sections, read_overlay: section table {dest,size,kind,entry},
ends on entry mismatch; kind==8 = BSS; plus LevelOverlay with .text words,
text_start and function starts.
- mask: hides relocated fields (j/jal targets, lui of main-RAM 0x0010..0x0040,
%lo/move/$gp completions tracked per-register).
- walk/HiState/address_refs/key: PS2 code walk with branch-delay-slot register
state.
- Relocation::new/same_code/copies/func/data: masked-code index, one-copy
lookup, data-address relocations.
A real-disc test asserts that a level-01 engine function's relocated copy is
found exactly once on all 26 other overlays (gate: every level-01 function is
reachable by masked-code identity on all 27).
Co-authored-by: reaper <reaper@users.noreply.github.com>