fix(ddsketch): make delta_transmission actually work end-to-end

[zzylol]

Summary

Three connected bugs that meant `delta_transmission: true` on the DDSketch processor silently behaved like full-state-only after the first window. End-to-end verification against the b3-delta config now shows three consecutive windows ingesting cleanly:

```
W1: OTLP modified-proto sketch ingest: 1000 routed, 0 decode-failed (full)
W2: OTLP modified-proto sketch ingest: 1000 routed, 0 decode-failed (delta)
W3: OTLP modified-proto sketch ingest: 1000 routed, 0 decode-failed (delta)
```

Pre-fix: W2/W3 logged `0 routed, 1000 decode-failed (fallback)` or `OTLP delta-sketch arrived before any base snapshot`.

Bugs

(1) Typed encoding always set to `Proto` (full)

```go
dp.SetEncoding(pmetric.DDSketchEncodingProto) // always — even for deltas
```

The other four sketch processors (CMS / CountSketch / HLL) already switch on the encoding string and emit the matching typed encoding. DDSketch was the outlier. The backend dispatches on `dp.encoding` (int), so deltas tagged as `Proto` landed in the proto_full decoder which tried to parse them as a `SketchEnvelope` and failed.

Fix: switch on the local `encoding` variable (already populated to `proto_full` / `proto_delta`) and call the matching `pmetric.DDSketchEncoding*` setter.

(2) `computeDDSketchDelta` was a stub returning full state

```go
func computeDDSketchDelta(snapPayload []byte, current *ddsketch.DDSketch, threshold uint64) ([]byte, error) {
_ = snapPayload
_ = threshold
return serializeDDSketch(current) // <-- always full state
}
```

Comment claimed sketchlib-go's delta encoder wasn't ready. It is — `sketches/DDSketch/delta.go` ships `ComputeDelta(snapshot, current, threshold) ([]byte, error)` producing proto bytes wire-compatible with the backend's `asap_otel_proto::sketchlib::v1::DdSketchDelta`.

Fix: deserialize `snapPayload` into a `*DDSketch` (`proto.Unmarshal` → `env.GetDdsketch()` → `ddsketch.NewFromState`), then call `ddsketch.ComputeDelta`.

(3) `ddsketch.encoding` attribute broke per-series snapshot lookup

Even with (1) + (2) fixed, deltas failed with `OTLP delta-sketch arrived before any base snapshot`. The backend caches snapshots keyed by the data point's full attribute set. The agent was adding:

```go
dp.Attributes().PutStr("ddsketch.encoding", encoding)
```

So full frames carried `…,ddsketch.encoding=proto_full,…` while delta frames carried `…,ddsketch.encoding=proto_delta,…` — different attribute set → different `series_key` → cache miss on every delta.

Fix: drop the attribute. The typed `Encoding()` field is the source of truth and what the backend dispatches on. Sibling processors don't carry the attribute either.

Companion required on backend side

Three full-state DDSketch envelopes for 1000 series exceeds the default tonic gRPC receive cap (4 MiB) — the gateway-to-backend export retries forever with `decoded message length too large: found 17103632 bytes`. Backend PR pending: bump `max_decoding_message_size` to 64 MiB to match what agent / gateway already do on their own receivers.

Test plan

• `build_sketchcollector.sh --skip-patches` clean.
• First window flush: backend decodes full state (`encoding=Proto`, 1000 routed).
• Second window flush: backend looks up per-series snapshot, applies delta, routes (`encoding=ProtoDelta`, 1000 routed, 0 decode-failed).
• Third window flush: same as second.
• PromQL query against the materialized data — separate residual issue (worker emits to store, but query-side lookup returns `No precomputed outputs found`; data is in the store per the runtime info endpoint). Independent of this PR.

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