refactor Emu/Config god objects: add OperatingSystem enum, persist ELF/Mach-O, remove dead fields

[brandonros]

Structural cleanup of the two largest structs (Emu, Config, SerializableEmu):

OperatingSystem enum replaces linux/macos bools

• Add OperatingSystem { Windows, Linux, MacOS } enum to arch/mod.rs
• Replace emu.linux: bool + emu.macos: bool with emu.os: OperatingSystem
• Windows is now an explicit variant instead of implicit !linux && !macos
• Update all 27 conditional sites across instruction_pointer, operands, syscall, svc, int handlers, and tests

Persist ELF/Mach-O structs on Emu (parity with PE)

• Add elf32: Option<Elf32>, elf64: Option<Elf64>, macho64: Option<Macho64> to Emu, matching the existing pe32/pe64 pattern
• Loaders now store parsed binary structs instead of dropping them after load
• Add addr_to_symbol: HashMap<u64, String> to Macho64 struct, populated during dylib loading — replaces the standalone emu.macho_addr_to_symbol

Remove dead fields

• Remove mnemonic: String — never written, actual mnemonic comes from ins.mnemonic(). Fix emu_context.rs to use instruction.mnemonic() instead
• Remove aarch64_instruction: Option<Instruction> — was set but never read

Organize structs with section comments

• Group Emu fields into: Configuration, Memory, Instruction decoding, Core execution, Platform & binary, Threads, API interception, Debugging, Tracing, Resource management
• Group Config fields into: Binary & arch, Memory layout, Heap behavior, Tracing, Verbosity, Console, Execution limits, API emulation, Threading, Inspection, Dump, Environment constants, Testing
• Group SerializableEmu to match Emu layout, with a dedicated "Flattened thread context" section for fields extracted at serialize time
• Add NOTE comments documenting fields intentionally not serialized
• Simplify SerializableEmu::Default to delegate to From<&Emu>

Nix flake

• Add flake.nix/flake.lock for reproducible Rust dev shell
• Per-system target lists (Linux vs macOS)
• Document nix develop in AGENTS.md