quest(relay): cluster routing with routes and upstream links

[kixelated]

Problem

Every MoQ node needs to route a subscription toward a broadcast's origin over whatever links it holds, with network changes costing messages per origin touched rather than per broadcast. One protocol has to serve moq.pro's edge and core tiers, customer clusters, and lossy drone meshes.

Approach

Parent branch for the cluster-routing questline in quest/m1/cluster-routing/README.md:

• Per-link upstream policy instead of edge/core roles (Upstream links), including tls:// qmux for edge links.
• A ROUTE per origin node plus path-less ANNOUNCE on one stream, loop-free by Babel feasibility (Routes and announces).
• One node id space with no cluster ids; boundaries between operators are Routing cost domains's.

It stays draft until every child lands, then the end-to-end tests and operator docs listed under Remaining work.

Decisions

• ✅ Realigned with main on 2026-10-03: merged main in, resolving in main's favor. inter-cluster.md (docs(quest): configure explicit cluster boundary identities #4718, docs(quest): cluster domains, not cluster ids, as boundary hops #4745) stays deleted; its operator-safety rules moved to the m3 routing cost domains quest in docs(quest): boundary identity constraints and TLS qmux prior art #4752.
• ✅ feat(relay): add edge and core cluster roles over TLS qmux #4731 (edge/core cluster.role over TLS qmux) closed; its tls:// qmux is prior art for Upstream links.
• ✅ Kept this PR and retitled it rather than closing and recreating it.

Impact

• Public API and wire: none in this parent. Child PRs describe their own impact.

Alternatives

• Link-state routing and the per-broadcast path vector were superseded (quest(m1): cluster routing as generic per-node routes and path-less announces #4694).
• Edge/core relay roles and cluster-domain hops were superseded by per-link upstream and one node id space.

Follow-ups

• Remaining work is tracked in quest/m1/cluster-routing/README.md.

🤖 Generated with Claude Code

(Written by Claude Opus 5.5)

[kixelated]

Closing: this questline branch carries no diff against main (its children landed on main or only touched quest files that are already there). Questline branches are being retired in favor of flat questlines (kixelated/quest#44); remaining children open their PRs against main.

(Written by Claude Opus 5.5)