Avoid duplicating the full piece buffer when parsing sampled-retrieval CARs

[SgtPooki]

readBlocks in the sampled-retrieval piece validation wraps the incoming piece buffer in new Uint8Array(pieceBytes) before handing it to CarReader.fromBytes:

dealbot/apps/backend/src/retrieval-anon/piece-validation.service.ts

Lines 192 to 200 in 2452e7d

	private async readBlocks(pieceBytes: Buffer, signal?: AbortSignal): Promise<SampledBlock[]> {
	const reader = await CarReader.fromBytes(new Uint8Array(pieceBytes));
	const blocks: SampledBlock[] = [];
	for await (const block of reader.blocks()) {
	signal?.throwIfAborted();
	blocks.push({ cid: block.cid, bytes: block.bytes });
	}
	return blocks;
	}

pieceBytes is a Buffer, which already is a Uint8Array, so that wrapper allocates a second full copy of the entire piece. Sampled pieces run up to ~500 MiB, so peak memory for one check is roughly 2x the piece size for no reason.

Fix: pass pieceBytes straight to CarReader.fromBytes(pieceBytes). Same behavior, drops the redundant allocation.

This is pre-existing from #487 and untouched by the rename in #612, so tracking it on its own. The file moves to apps/backend/src/sampled-retrieval/piece-validation.service.ts once #612 lands.