FreeBSD native target (amd64) — raw-syscall, stays in the libc-free family

• Type: feature (Track A — backend/ABI/ELF/syscall emission). Portability campaign.
• Status: backlog
• Owner: —
• Opened: 2026-07-17, OS-portability mapping session. Full map in devdocs/dev/portability-axes.md.
• Related: [[feature-port-rtl-over-libc]] (NOT needed here — FreeBSD keeps raw syscalls), [[feature-port-openbsd-libc]] (the libc-through sibling). North star: [[ir-as-substrate]].

Why FreeBSD is the cheapest first win

Same family as Linux: raw-syscall ELF on amd64, identical argument registers (rdi/rsi/rdx/r10/r8/r9). Keeps the libc-free identity intact. And FreeBSD's linuxulator (linux64.ko) runs today's Linux pxx binaries unmodified — a zero-work smoke on a real BSD kernel before writing a line of code.

What differs from Linux (the whole delta)

1. Syscall numbers — FreeBSD's own table (write=4, exit=1, …), not Linux's.
2. Error convention — FreeBSD signals an error via the carry flag (CF set, errno in rax), NOT Linux's negative-rax. This is the real work — every syscall wrapper's error check changes, not just a number swap.
3. ELF brand — set EI_OSABI = ELFOSABI_FREEBSD (9) or emit the NT_FREEBSD_ABI_TAG note so the kernel brands the binary correctly.
4. exit_group → exit (no thread-group exit; adjust the process-exit path).

Plan

• Add the FreeBSD syscall-number table + carry-flag error path behind the platform axis (see [[project_pal_platform_axis_step1]]); --platform=freebsd.
• Brand the ELF in elfwriter.inc.
• Smoke order: (a) linuxulator runs a current Linux binary (no build change); (b) native --platform=freebsd hello-world; (c) heap/string/exception torture.

Acceptance

• --platform=freebsd emits an amd64 ELF that runs natively on FreeBSD (qemu image) and produces output byte-identical to the Linux build for a scalar + heap/string/exception torture program.
• Linuxulator smoke logged (runs a current Linux pxx binary on FreeBSD) as the pre-native checkpoint.
• Gate: make test + self-host byte-identical (Linux default unchanged); FreeBSD run under qemu.

Test infra

qemu FreeBSD image (pre-built qcow2 exists) + linuxulator. The per-OS image/runner harness may live in a Track T clone (see portability-axes.md) — the compiler work is here.