refactor(mrtr): split experimental module into package

mrtr.py → mrtr/
├── __init__.py  — package docstring + re-exports
├── _state.py    — encode_state/decode_state + input_response helper
├── context.py   — MrtrCtx (Option F, ship target)
├── builder.py   — ToolBuilder (Option G, ship target)
└── compat.py    — sse_retry_shim + dispatch_by_version (comparison artifacts)

Ship targets (F/G) now live separately from the dual-path compat shims.
All imports from mcp.server.experimental.mrtr unchanged.

Author: maxisbey