feat(control_plane): emit cold-archive intchunk format to the edge agent

[zzylol]

Summary

• Add a cold-archive format knob so the control plane can tell the edge agent to ship the lossless intchunk cold-part format instead of the default gorilla-XOR fragments. The agent's asapedgeprocessor already supports both (cold.format: fragment → /ingest/gorilla, cold.format: intchunk → cold.coldpart_endpoint /ingest/coldpart); the control plane previously had no way to emit format:/coldpart_endpoint:, so the agent always got fragments.
• New EdgeStageConfig fields: cold_format (enum fragment default | intchunk) and cold_coldpart_endpoint: Option<String> (in physical/colored_dag/emitter.rs), with named defaults default_cold_coldpart_endpoint() + a coldpart_endpoint_from_ship() deriver. Both #[serde(skip_serializing_if ...)] so an unset format is byte-identical on the wire.
• The fused asap_edge cold-block builder (emit/stage_config.rs::emit_edge_yaml_asap_edge) now writes format: intchunk + coldpart_endpoint: only when cold_format == Intchunk. The default fragment path emits neither key.
• Operator SET path: ASAP_COLD_FORMAT=intchunk env knob, read in the shared bootstrap/replan stitch (emit/mod.rs::apply_cold_format_from_env, invoked from extend_edge_with_demo_plumbing) — mirrors how ASAP_CLUSTER drives the cold external-label default. The coldpart endpoint is derived from the fragment ship endpoint (same merger host:port, /ingest/coldpart) unless ASAP_COLD_COLDPART_ENDPOINT overrides it.
• Default stays fragment — no behavior change when unset. Data plane / warm path untouched.

Test plan

• cargo build -p control_plane green (no new warnings).
• cargo test -p control_plane green: 779 lib + 28 bin pass.
• New emit tests: cold_format_default_fragment_emits_no_format_keys (fragment → no format:/coldpart_endpoint: key), cold_format_intchunk_emits_format_and_derived_coldpart_endpoint (intchunk → both keys, endpoint derived), cold_format_intchunk_honours_explicit_coldpart_endpoint, and cold_format_env_knob_opts_into_intchunk_and_derives_endpoint (the ASAP_COLD_FORMAT SET path).

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