The runtime already knew how to spawn stdio MCP processes, but it still
needed transport primitives for framed JSON-RPC exchange. This change adds
minimal request/response types, line and frame helpers on the stdio wrapper,
and an initialize roundtrip helper so later MCP client slices can build on a
real transport foundation instead of raw byte plumbing.
Constraint: Keep the slice small and limited to stdio transport foundations
Constraint: Must verify framed request write and typed response parsing with a fake MCP process
Rejected: Introduce a broader MCP session layer now | would expand the slice beyond transport framing
Rejected: Leave JSON-RPC as untyped serde_json::Value only | weakens initialize roundtrip guarantees
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Preserve the camelCase MCP initialize field mapping when layering richer protocol support on top
Tested: cargo fmt --all --manifest-path rust/Cargo.toml
Tested: cargo clippy -p runtime --all-targets --manifest-path rust/Cargo.toml -- -D warnings
Tested: cargo test -p runtime --manifest-path rust/Cargo.toml
Not-tested: Integration against a real external MCP server process
8b6bf4cee7