feat: coldpart Part format in asap-gorilla-go + agent intchunk cold-part ship path

[zzylol]

Summary

First of a two-PR change (merge this one first; the ASAPQuery-backend re-import PR follows).

• Relocate coldpart to the shared lib. Adds asap-gorilla-go/coldpart (full read+write code + tests), next to intchunk, so BOTH the edge agent (producer) and the backend gorilla-merger (consumer) can import the cold "Part" format. The merger's internal/coldpart is deleted in the follow-up backend PR.
• Agent intchunk cold-part path (opt-in, default OFF). Adds a parallel cold ship path in the asap_edge processor. When cold.format: intchunk (default is the existing gorilla-XOR fragment path, unchanged), the SAME per-window drained samples are re-encoded as a lossless coldpart.Part with absolute-ms block bounds and POSTed to the merger's POST /ingest/coldpart (cold.coldpart_endpoint). The merger derives the content-addressed key itself; the agent sends only the bytes.
• Shared helpers. New exported gorilla.FragmentLabels (single source of truth for fragment→labels, also used by the fragment→TSDB finalizer) guarantees a series is queried under one identity regardless of cold format; gorilla.DecodeFragmentSamples recovers raw samples from the same drained XOR fragments.

The flag is wired through ColdConfig (the same config path that selects the current cold ship behavior); an operator typically sets it from an env var, e.g. cold.format: ${env:ASAP_COLD_FORMAT} with ASAP_COLD_FORMAT=intchunk.

Test plan

• asap-gorilla-go: go build ./..., go test ./... (incl. moved coldpart tests), go vet, gofmt all clean.
• asapedgeprocessor: builds + full go test ./... green; go vet/gofmt clean.
• New integration tests: agent builds a part → coldpart.OpenPart + Series() round-trips samples bit-exactly with absolute-ms block bounds; POST to an in-process /ingest/coldpart-style handler validates via OpenPart; full ConsumeMetrics→flushAll exercises the opt-in path end to end.
• Default-off proof: with cold.format unset the processor keeps the fragment path and wires no cold-part shipper (no behavior change).

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