Add a standard-library system capability matrix

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.

Start by reviewing the current std::sys modules and the tracking issues #375, #367, and #370. Build a matrix separating compiler target support from standard-library source coverage, record each listed capability with explicit support states, link the existing issues, and mark unsupported platforms clearly.

Written by the indexing model from the issue text.