Add target-gated signal primitives to std::sys

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.

Start with tracked issue #375 and the representation in #378, then review the std::sys signal boundary and target-specific ABI requirements. Document callback layouts and compiler limitations before implementation; completion requires Linux and macOS install, delivery, and restore tests that do not hang CI, plus clear failures for unsupported targets.

Written by the indexing model from the issue text.