Add a FreeBSD std::sys platform foundation

[LunaStev]

Tracked by #375 and represented in #378.

Goal

Create a source-auditable FreeBSD system-provider foundation even before full compiler and runtime target support is advertised.

Scope

• Define target-gated FreeBSD module structure for fs, process, memory, time, environment, sockets, terminal, randomness, and signals.
• Record syscall numbers, structures, constants, and pointer widths from authoritative FreeBSD headers and documentation.
• Keep architecture-specific details separated for x86-64 and AArch64.
• Add parser and semantic validation fixtures that can run before backend support.
• Gate runtime tests until the compiler target and CI environment exist.

Completion criteria

• FreeBSD source contracts do not affect supported Linux or macOS builds.
• Every constant and ABI layout has an authoritative source note.
• Unsupported runtime status is explicit in diagnostics and Add a standard-library system capability matrix #378.
• Backend target enablement remains a separate issue.
• Runtime smoke tests are ready to activate when a FreeBSD runner is available.