docs: document stateless dual-protocol support

[lucarlig]

Summary

• define the three canonical product components:
  • ContextForge control plane: Python management plane in IBM/mcp-context-forge
  • ContextForge built-in dataplane: Python MCP request path in the same repository, supporting old/new protocols and stateful/stateless behavior
  • ContextForge external dataplane: this independently deployable Rust repository
• use the canonical names throughout the wiki and remove ambiguous local, slow, and fast dataplane labels
• document direct stateless external-dataplane support for MCP 2026-07-28 and 2025-11-25
• define 2026-07-28 → 2025-11-25 and 2025-11-25 → 2026-07-28 as best-effort, sessionless adaptation paths
• retain initialize as a stateless external-dataplane compatibility request
• add the four-combination matrix and a brief tools/call implementation note linked to MCP tool/call handling IBM/mcp-context-forge#6327
• align topology, deployment, security, testing, operations, and contributor guidance with the component model
• fix Rust 1.98 unused_async_trait_impl failures by returning immediately-ready futures from synchronous trait handlers

Validation

• mdbook build _context/wiki
• mdbook test _context/wiki
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo nextest run --locked --workspace (223 passed, 3 skipped)
• cargo +1.96.1 check --locked --workspace --all-targets
• pre-commit hooks under the repository MSRV toolchain (RUSTUP_TOOLCHAIN=1.96.1)