Document supported targets and host runtime behavior

[lukewilliamboswell]

Context

The implementation has a clear five-target matrix in platform/main.roc:84-90 and CI now builds and runs:

• x64 macOS;
• arm64 macOS;
• x64 Linux musl;
• arm64 Linux musl;
• x64 Windows.

The public README does not present this support matrix. CONTRIBUTING.md:60 is also stale: it says arm64 Linux is compile-only, while .github/workflows/ci.yml:130-131 now runs it on an arm64 runner.

Several host behaviors are important for users but currently require reading Rust/Cargo source, including:

• HTTPS uses bundled WebPKI roots rather than the OS trust store;
• the current HTTP implementation enables HTTP/1;
• TCP operations currently have no timeout controls;
• SQLite connections are cached by path;
• Windows support is x64 only.

There are also small documentation typos such as “subsitition” in Cmd.roc and “bevahiour” in Tty.roc/Stdin.roc.

Suggested direction

Add a concise support/runtime section to README or contributor-facing documentation and ensure generated module docs link to it where useful. Keep it factual and update it as the networking/SQLite issues from the release audit are resolved.

Acceptance criteria

• README documents all supported OS/architecture targets.
• The stale arm64 Linux compile-only statement is corrected.
• TLS trust-store and HTTP protocol behavior are documented.
• Important TCP and SQLite lifecycle/timeout semantics are documented according to their final implementations.
• Known platform-specific limitations, including Windows architecture support, are listed.
• Obvious documentation typos found during the audit are corrected.