feat(arm64): fix rmcp-sdk compatibility and add arm64 documentation

[w8explorer]

Summary

This PR enables building and running OpenFang on ARM64 (aarch64) architectures. It addresses a critical compilation error in the rmcp SDK and provides a dedicated deployment guide for ARM-based cloud environments.

Changes

Core Fix: Refactored crates/openfang-runtime/src/mcp.rs to use Default::default() for StreamableHttpClientTransportConfig. This resolves error E0639 caused by the #[non_exhaustive] attribute in the latest rmcp library.
Documentation: Added docs/arm64.md detailing native system dependencies and build instructions for aarch64 Linux.
Visibility: Added an "ARM64 Support" link to the main README.md header.

Testing

cargo clippy --workspace --all-targets -- -D warnings (Verified on ARM64).
cargo check -p openfang-runtime passes on Neoverse-N1 architecture.
Live integration tested on Oracle Cloud ARM64 instance.

Security

No new unsafe code.
No secrets or API keys in diff.
User input validated at boundaries.

Technical Assistance: Code debugging, architecture verification, and documentation were assisted by Antigravity (an AI by Google DeepMind).

[jaberjaber23]

Closing — the core mcp.rs fix (switching from struct literal to Default::default() for StreamableHttpClientTransportConfig) is already addressed by #997 (which uses the builder pattern instead). The benchmarks/ directory and docs/arm64.md are unrelated to the rmcp fix and add maintenance burden. If you'd like to contribute the ARM64 documentation separately, please open a focused PR without the benchmark scripts.