(DON'T MERGE) refactor(net): replace routes instead of splicing

[kixelated]

Summary

• Replace the synthetic route-splicing broadcast with direct per-consumer source selection.
• When a different shortest or tied source wins, emit an unannounce followed by an announce. Existing subscriptions remain bound to their original source.
• Remove the resume and partial-group handover machinery, and update the Rust/JS adapters, relay behavior, CLI guidance, documentation, and normative drafts.

The root cause was that route selection mutated a synthetic broadcast underneath live subscribers. Preserving subscription state across independently owned sources required frame-precise resume bookkeeping and several handover paths, which repeatedly produced lifecycle and routing bugs. Source-bound subscriptions make replacement explicit and leave resubscription policy with the consumer.

Public API changes

• No exported items were renamed, removed, or signature-changed.
• Behavioral change: a newly selected source is observable as an unannounce/announce replacement, including for a shorter or tied route. Consumers that want continuity must subscribe to the replacement announcement.

Test plan

• nix develop --command just fix
• nix develop --command cargo nextest run -p moq-net (712 passed)
• Focused moq-relay GOAWAY replacement tests
• nix develop --command just drafts check
• nix develop --command just check
• nix develop --command just test smoke-full (21 publisher/subscriber combinations passed)
• git diff --check

(Written by GPT-5)

[kixelated]

I made this as an emergency break glass PR.