Add the memnet-mcp product gateway (MN-REQ-06.12)

[chouswei]

SysML first, then the memnet-mcp capability that follows it. No SemVer bump. Not merged. Not deployed.

Requirement

• MN-REQ-06.12 (MN_REQ_06_12_ProductGateway / productGatewayReq)
• MN-VER-06-S10 (MN_VER_06_S10_ProductGateway)
• Part MemNetProductGateway nested on MemNetLanMcpFront. Parent catalogue stays inventOnly=true, implemented=false, codeApproved=false (Research: one MCP server in front of several MemNet serves (LAN cluster) #191). Child is implemented=true.

Today (unchanged by this cut)

Checked on tags v0.19.6 (droplet memnet-mcp-http) and v0.19.18 (Pi serve).

1. One MCP, one serve. Neither tag can point one MCP at several serves. TCP uses MEMNET_SERVE_HOST / MEMNET_SERVE_PORT only (v0.19.6 client.py 105–139, config.py 90–95; v0.19.18 client.py 106–140, config.py 96–101).
2. Auth in front. streamable-http optional exact MEMNET_MCP_HTTP_TOKEN (http_transport.py 70–76 and 230–264, both tags). stdio has none. caller is optional CapsPolicy when session ACL is on. Tip mn_tip_ exists only at v0.19.18 (portal SHA-256 + nginx), not per-product, and is not accepted as a product credential.
3. Wire. A v0.19.6 MCP can talk to a v0.19.18 serve for the argv it sends. Envelope is {args, stdin?} → {exit_code, stdout, stderr}. v0.19.18 adds optional admin_token / admin_usage, --max-edges, --product, --token, admin usage-report, and session expire-status. The reverse breaks: a v0.19.18 MCP always sends --max-edges, which v0.19.6 CLI lacks.

This cut

memnet-mcp --transport gateway when MEMNET_GATEWAY_CONFIG is set:

• backend registry (id → host:port; each product's backends, houses, pinned version)
• route by product, house, or session; one owning serve; no silent move
• hashed, scoped, rotatable, revocable per-product credentials
• product namespace isolation for session open, load, save, close, and list
• exact pass-through of GQL, stdin, and engine @ERR / @WRN / @STAT
• @ERR: gateway_<code>|… refusals
• health check; failover only for a new session with no house and no backend; otherwise gateway_backend_unreachable
• version check against the pin
• loopback or tailnet bind only unless MEMNET_GATEWAY_ALLOW_PUBLIC=1
• per-product admin counts (no content, no session ids)
• stdio and streamable-http ignore the registry

Contract: docs/operations/product-gateway-contract.md (implemented for this cut; not a draft).

Tests run two real memnet serve processes, including a 0.19.18-compatible wire.

Endleaf

Backend endleaf-rpi5-syson at tailnet 100.118.79.40:18795, product endleaf, house syson, pin 0.19.18. Sample JSON and the hash command are in the contract. The droplet's existing 0.19.6 memnet-mcp-http does not speak this contract.

Open

See the contract, decisions 1–12: agent MCP tools do not use the registry; owner file is one process; counts reset on stop; list keeps backend max; unknown and foreign sessions share gateway_forbidden_session; no automatic return and no SnapshotHandCarry; public bind is opt-in; MagicDNS is not resolved; mn_tip_ is not federated; version cache default 15s; empty-paren edge delete is an engine lower, not rewritten; sessions opened outside this gateway are not adopted.