feat(runtime/darwin): arm64 entry via LC_MAIN register convention

[octalide]

Part of briar-systems/mach#1718 (the dynamic/PIE darwin epic). Pairs with the mach PIE-writer child (briar-systems/mach#1722).

What

mach is moving darwin arm64 executables to PIE LC_MAIN images (required to exec on Apple Silicon — proven in mach#1718). LC_MAIN hands control to a C-main-style entry with arguments in registers, not on the stack. This rewrites the darwin arm64 _start to capture argc/argv/envp from x0/x1/x2 (the LC_MAIN register ABI dyld enters with) instead of reading them off [sp].

Everything else is unchanged: still stores into _rt_argc/_rt_argv/_rt_envp, calls _rt_init then main, exits via raw svc 0x80 (no libSystem). The x86_64 darwin entry is untouched (it keeps the LC_UNIXTHREAD stack convention until the paired mach writer switches it too).

Validation

The darwin runtime is cross-compile-only on the Linux CI host, so this is validated end-to-end on the mach side: the paired PIE writer (mach#1722) cross-builds an aarch64-darwin compiler and execs it on a macos-14 runner (no Killed: 9) where it self-hosts. mach-std CI stays green (the darwin arm64 file is $if-guarded off non-darwin builds).

Closes #324

🤖 Generated with Claude Code

[octalide]

Added a second commit to this branch: fix(os/darwin): correct mkdirat/unlinkat/faccessat syscall numbers.

mkdirat was 464 (that number is openat_nocancel), unlinkat was 466, faccessat was 468 — all wrong. The authoritative XNU values are mkdirat=475, unlinkat=472, faccessat=466. These only surfaced once arm64 darwin binaries actually run (the mach PIE writer, briar-systems/mach#1722): the native self-host build on a macos-14 runner failed at "failed to create object directory" because mkdirat hit the wrong trap. With the fix, the native a→b→c self-host fixpoint converges on Apple Silicon and mach test . runs (425/426 pass).

These are latent darwin-runtime bugs that were never exercised before (darwin binaries couldn't run); folding them in here keeps all the "make arm64 darwin actually run" runtime work in one PR. Happy to split into a separate PR if preferred.