Add target-gated signal primitives to std::sys

[LunaStev]

Tracked by #375 and represented in #378.

Scope

Define the minimum raw signal contract required by runtimes and applications:

• Stable signal constants by target.
• sigaction-compatible handler registration where Wave can express the ABI safely.
• Signal mask inspection and updates.
• raise or kill-based delivery.
• Default, ignore, and restore behavior.
• Explicit restrictions for async-signal-safe Wave operations.

Start with Linux and macOS. Record Windows console-control and exception handling as a separate provider contract rather than forcing Unix semantics onto Windows.

Completion criteria

• ABI layouts and callback signatures are documented before implementation.
• Linux and macOS constants are target-specific and source-auditable.
• Compiler limitations around callbacks or function pointers are linked.
• Runtime tests cover install, deliver, and restore without hanging CI.
• Unsupported targets fail clearly.