Add a standard-library system capability matrix

[LunaStev]

Tracked by #375. Related to #367 and #370.

Problem

Compiler target support and std::sys source coverage are separate concerns, but the repository does not show that distinction. This makes it difficult to add platform contracts ahead of executable backend support.

Scope

Create a capability matrix covering at least:

• Linux x86-64 and RV64.
• macOS x86-64 and AArch64.
• Windows x86-64 and AArch64.
• FreeBSD x86-64 and AArch64.
• WASI.

For each platform, record the status of fs, process, memory, time, environment, sockets, terminal, randomness, signals, and startup support.

Use explicit states such as implemented and runtime-tested, source-only contract, planned, and unsupported. Do not claim runtime support from source presence alone.

Completion criteria

• Compiler target support and std source coverage have separate columns.
• Every current std::sys module appears in the matrix.
• Existing tracking issues are linked.
• Unsupported platforms are clearly marked.
• Adding a new capability requires a small, documented matrix update.