docs(solana-rpc): expand agave and geyser runbook

doc/step-002d-snapshot-download-mainnet.txt

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+Step 002D: Download snapshot (mainnet) do /var/lib/solana/ledger (wymagane na mainnet)
+
+Objaw (bez snapshot):
+  Failed to start validator: ... did you forget to provide a snapshot
+
+UWAGA:
+- Snapshot full jest DUZY (setki GB). Ten krok moze trwac dlugo.
+- Pobieramy full + incremental z oficjalnego endpointu (api.mainnet-beta.solana.com).
+
+0) Stop validator
+  sudo systemctl stop agave-validator
+
+1) (Opcjonalnie, polecane) wyczysc stare dane ledger (zostaw genesis.*)
+  sudo -u solana -H bash -lc '
+    cd /var/lib/solana/ledger
+    rm -rf rocksdb snapshots accounts_index accounts_hash_cache admin.rpc ledger.lock
+    ls -la | head
+  '
+
+2) Pobierz FULL + INCREMENTAL snapshot do ledger (jako user `solana`)
+  sudo -u solana -H bash -lc '
+    set -euo pipefail
+    cd /var/lib/solana/ledger
+
+    FULL_REL="$(curl -sI https://api.mainnet-beta.solana.com/snapshot.tar.bz2 | awk "/^location:/{print \\$2}" | tr -d "\\r")"
+    INC_REL="$(curl -sI https://api.mainnet-beta.solana.com/incremental-snapshot.tar.bz2 | awk "/^location:/{print \\$2}" | tr -d "\\r")"
+
+    echo "FULL_REL=$FULL_REL"
+    echo "INC_REL=$INC_REL"
+
+    FULL_URL="https://api.mainnet-beta.solana.com${FULL_REL}"
+    INC_URL="https://api.mainnet-beta.solana.com${INC_REL}"
+
+    FULL_FILE="$(basename "$FULL_REL")"
+    INC_FILE="$(basename "$INC_REL")"
+
+    echo "Downloading FULL: $FULL_FILE"
+    curl -fSL "$FULL_URL" -o "$FULL_FILE"
+    ls -lh "$FULL_FILE"
+
+    echo "Downloading INC:  $INC_FILE"
+    curl -fSL "$INC_URL" -o "$INC_FILE"
+    ls -lh "$INC_FILE"
+  '
+
+3) Start validator + logi
+  sudo systemctl start agave-validator
+  sudo journalctl -u agave-validator -n 80 --no-pager
+