Define OpenBSD and NetBSD target contracts beyond FreeBSD

## Goal

Define compiler, standard-library, and CI contracts for the OpenBSD and NetBSD roadmap suites introduced by #457. FreeBSD compiler work remains tracked separately in #413.

## Scope

- Select exact supported triples and architecture subsets for each OS.
- Define ELF ABI, linker, CRT, libc, syscall-provider, and sysroot contracts separately for OpenBSD and NetBSD.
- Add object-level cross checks before introducing native VM runners.
- Keep OS capability differences explicit in std providers and diagnostics.
- Split per-OS implementation issues once the shared contract inventory is complete.

## Completion criteria

- A documented target/support table distinguishes OpenBSD, NetBSD, and FreeBSD.
- Object and C ABI fixtures exist for the first selected architecture on each OS.
- Missing SDK/sysroot failures are structured and actionable.
- Matrix cells remain disabled until their claimed level is verified.

Related: #413, #457

Start by reading related issues #457 and #413, then inventory the compiler, standard-library, ABI, sysroot, syscall-provider, and CI contracts described here for OpenBSD and NetBSD. Define the supported triples and first architectures before splitting per-OS implementation issues. Done means the support table, object and C ABI fixtures, structured SDK/sysroot failures, and verified matrix gating are documented.

Written by the indexing model from the issue text.