replace xmas-elf with object

`xmas-elf v0.10.0` and its transitive dep `zero v0.1.3` are used for ELF parsing in: - `kernel/src/mem/<mod.rs>` - `kernel/src/backtrace/<symbolize.rs>` - `loader/src/<kernel.rs>` - `loader/src/<mapping.rs>` ## Why remove them - `zero v0.1.3` is a tiny, essentially unmaintained crate (last meaningful update ~2016). - `xmas-elf` is narrow in scope and not actively maintained. - The [`object`](<https://crates.io/crates/object>) crate is the modern, well-maintained alternative for ELF/object file parsing. It is already an indirect dependency (via `backtrace` in xtask) and is used by `gimli`/`addr2line` which we already depend on. ## Suggested approach Replace `xmas_elf::ElfFile` and related types with the `object` crate API. Both `object::File` and the section/symbol iterators cover all current xmas-elf usage. ## Impact Removes 2 crates (`xmas-elf`, `zero`). Consolidates around the same ELF library already used in the broader ecosystem of our existing deps.