fix(evnode-fibre): cap per-block data at 100 MiB to fit a Fibre upload

Companion to the submitter chunking fix. The submitter can
split a multi-blob batch into ≤120 MiB Fibre uploads, but
a *single* block_data item that exceeds 128 MiB still ends
up alone in its own chunk and fails server-side ("blob size
exceeds maximum allowed size"). Lower the per-block cap to
100 MiB so under high-throughput txsim a single block can't
grow past Fibre's hard limit, and update the comment to
explain the relationship between this cap and Fibre's
~128 MiB upload reject threshold.

Author: walldiss

tools/celestia-node-fiber/cmd/evnode-fibre/main.go

@@ -302,7 +302,12 @@ func run(cli cliFlags) error {
 	// Fiber-tuned profile: BatchingStrategy=adaptive, BatchMaxDelay=1.5s,
 	// DA.BlockTime=1s, MaxPendingHeadersAndData=0, plus 120 MiB blob cap.
 	cfg.ApplyFiberDefaults()
-	block.SetMaxBlobSize(120 * 1024 * 1024)
+	// 100 MiB — bounded by Fibre's hard ~128 MiB per-upload cap (we
+	// hit `data size exceeds maximum 134217723` at 128 MiB - 5 B).
+	// Set the per-block data cap below that so each block_data item
+	// fits in a single Fibre upload after the submitter splits a
+	// multi-blob batch into ≤120 MiB chunks.
+	block.SetMaxBlobSize(100 * 1024 * 1024)
 	cfg.P2P.ListenAddress = cli.p2pListen
 	cfg.P2P.DisableConnectionGater = true
 	cfg.RPC.Address = cli.rpcListen