fix(celestia-node-fiber): report original payload size in BlobEvent

[walldiss]

Summary

Follow-up to #3280 (landed a few seconds before this fix was pushed). Corrects the `BlobEvent.DataSize` reported by the adapter's Listen path.

The bug

`listen.go` set `BlobEvent.DataSize` to `b.DataLen()`. For a share-version-2 Fibre blob, `DataLen()` is always the fixed on-chain share layout — `fibre_blob_version (4 bytes) + commitment (32 bytes)` — not the original payload length.

The 10-blob showcase on julien/fiber asserts `DataSize > 0` because of this, but ev-node's `fibermock` contract is `DataSize = len(data)` (original payload length). Any downstream consumer using `DataSize` to pre-allocate buffers, filter by size, or report progress would be silently misled.

Why Download-to-resolve

The v2 share genuinely doesn't carry the original size, and `x/fibre` v8 exposes no query that maps `commitment → PaymentPromise.BlobSize`. Events don't carry size either (`EventPayForFibre` has signer/namespace/commitment/validator_count — no size). So the only accurate path is to Download the blob and measure. Listen now does that per v2 event before forwarding.

Cost: one FSP round-trip per v2 BlobEvent. For the showcase (10 × ~40 bytes) this is noise. For 128 MiB blobs it's non-trivial — we can add an opt-out config switch if that becomes material. Correctness wins for now.

Changes

• `listen.go` — `forwardFibreBlobs` and `fibreBlobToEvent` become methods on `*Adapter` so they can reach `a.fibre.Download`. `fibreBlobToEvent` issues a Download, reads `len(res.Data)` for `DataSize`. Doc on `Listen` explains the tradeoff.
• `adapter_test.go` — `TestListen_FiltersFibreOnlyAndEmitsEvent` stubs `fakeFibre.Download` with a deterministic payload and asserts `DataSize == len(payload)`.
• `testing/showcase_test.go` — restored the strict `DataSize == len(expected[key])` assertion for all 10 blobs.

Test plan

• `go build -tags fibre ./...`
• `go vet -tags fibre ./...`
• `go test -count=1 ./...` — 7 adapter unit tests pass (no tag needed)
• `go test -tags fibre -count=1 ./...` — 7 unit + 1 showcase (10 blobs, each with exact `data_size` match) — all pass in ~10s

Sample output after the fix

```
upload[00] size=37 → listen[01] data_size=37
upload[01] size=38 → listen[02] data_size=38
...
upload[09] size=46 → listen[10] data_size=46
```

(On julien/fiber pre-fix: all 10 reported `data_size=36`.)

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