control-plane: thread cold ship_endpoint + external_labels into EdgeStageConfig

[zzylol]

Summary

Follow-up to #311. The merged emit_edge_yaml_asap_edge derived the fused asap_edge cold tier target from a placeholder: ship_endpoint came from the OTLP backend host (http://<backend>:9098/ingest/gorilla) and external_labels.cluster was read inline from ASAP_CLUSTER — because EdgeStageConfig carried neither field. Both were wrong: the cold tier ships to the gorilla-merger over HTTP ingest port 10908 (gRPC 10907), not backend:9098.

• Add cold_ship_endpoint: Option<String> and cold_external_labels: Vec<(String, String)> to EdgeStageConfig (physical/colored_dag/emitter.rs).
• Add two named defaults in one place — default_cold_ship_endpoint() (http://gorilla-merger:10908/ingest/gorilla) and default_cold_external_labels() (cluster=<ASAP_CLUSTER|asap-mvp>) — and route both the emit fallback and the L5 build site through them (no more inline guess).
• emit_edge_yaml_asap_edge now reads the threaded fields (falling back to the named defaults when unset). ASAP_EDGE_FUSED gating untouched.
• Populated the new fields at every EdgeStageConfig struct-literal site (~18 across 6 files) so the crate compiles. The colored_dag L5 build site uses the named defaults because that layer is deployment-independent (no DeploymentConstraints plumbed in — reported, not fabricated); a deploy-aware layer can overwrite edge.cold_* post-emit the same way exporter_target is.

Threading-from-plan note

The colored_dag L5 emitter is explicitly deployment-independent — it has no access to a real per-deploy cold endpoint or cluster name. So the L5 site populates the named defaults (gorilla-merger:10908, cluster=asap-mvp), not a plan-resolved value. The new fields make it trivial for a future deploy-aware caller to thread real values; this PR fixes the wrong host/port and removes the inline placeholder.

Test plan

• cargo build -p control_plane clean — no new warnings vs origin/main baseline (lib 4 / bin 2 warnings, identical set, all pre-existing).
• cargo test -p control_plane --lib emit::stage_config — 84 pass.
• cargo test -p control_plane --lib physical::colored_dag — 21 pass.
• Extended fused_asap_edge_emits_single_pipeline_and_metrics_list to thread cold_ship_endpoint = http://gorilla-merger:10908/ingest/gorilla + cluster=asap-mvp and assert the emitted cold block surfaces exactly that (and never emits :9098).
• Full --lib suite passes single-threaded (769/769). NOTE: the full parallel --lib run flakes on a pre-existing ASAP_EDGE_FUSED process-global std::env race introduced by control-plane: emit fused asap_edge edge config #311 (its fused emit test sets the env var while parallel mvp46 tests read it without the shared lock) — reproduced identically on the untouched origin/main baseline (5/8 parallel failures there). Out of scope for this PR; not caused by this change.