feat(pumpfun): runtime V2 flag, buyback fee pool fix, legacy ix encoding

- Replace compile-time `pumpfun-v2` feature flag with runtime
  `TradeConfig::use_pumpfun_v2(bool)` for flexible V1/V2 switching
- Fix BUYBACK_FEE_RECIPIENTS pool: replace wrong addresses (was reusing
  standard pool + FEE_CONFIG) with official buyback pool from
  FEE_RECIPIENTS.md
- Fix legacy buy/buy_exact_sol_in ix data encoding: use 2-byte
  Option<bool> (option tag + value) matching official Pump SDK, 26 bytes
  total instead of broken 25
- Add `SwapParams.use_pumpfun_v2` field and wire through TradingClient
  buy/sell paths
- V2 instructions read `quote_mint` from `PumpFunParams` (not
  BondingCurveAccount which lacks the field)
- Fix `fetch_bonding_curve_account` to use BorshDeserialize instead of
  non-existent `decode_from_chain_account_data`
- Update all examples with `use_pumpfun_v2: false` field
- Update README with unified V1/V2 section showing both enabling methods

examples/cli_trading/src/main.rs

@@ -636,6 +636,7 @@ async fn handle_buy_pumpfun(
         gas_fee_strategy: gas_fee_strategy,
         simulate: false,
         use_exact_sol_amount: None,
+            use_pumpfun_v2: false,
         grpc_recv_us: None,
     };
     match client.buy(buy_params).await {
@@ -692,6 +693,7 @@ async fn handle_buy_pumpswap(
         gas_fee_strategy: gas_fee_strategy,
         simulate: false,
         use_exact_sol_amount: None,
+            use_pumpfun_v2: false,
         grpc_recv_us: None,
     };
     match client.buy(buy_params).await {
@@ -748,6 +750,7 @@ async fn handle_buy_bonk(
         gas_fee_strategy: gas_fee_strategy,
         simulate: false,
         use_exact_sol_amount: None,
+            use_pumpfun_v2: false,
         grpc_recv_us: None,
     };
     match client.buy(buy_params).await {
@@ -808,6 +811,7 @@ async fn handle_buy_raydium_v4(
         gas_fee_strategy: gas_fee_strategy,
         simulate: false,
         use_exact_sol_amount: None,
+            use_pumpfun_v2: false,
         grpc_recv_us: None,
     };
     match client.buy(buy_params).await {
@@ -869,6 +873,7 @@ async fn handle_buy_raydium_cpmm(
         gas_fee_strategy: gas_fee_strategy,
         simulate: false,
         use_exact_sol_amount: None,
+            use_pumpfun_v2: false,
         grpc_recv_us: None,
     };
     match client.buy(buy_params).await {