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

src/trading/core/params/dex_swap.rs

@@ -97,6 +97,10 @@ pub struct SwapParams {
     /// When Some(false), uses regular buy instruction where slippage is applied to SOL/quote input.
     /// This option only applies to PumpFun and PumpSwap DEXes; it is ignored for other DEXes.
     pub use_exact_sol_amount: Option<bool>,
+    /// Use PumpFun V2 instructions (buy_v2 / sell_v2 / buy_exact_quote_in_v2, 27-account metas, quote_mint support).
+    /// Default: `false` — uses V1 instructions (18-account metas, legacy SOL-paired).
+    /// Set to `true` when PumpFun officially deploys V2 on mainnet. Until then, keep `false` for smaller transaction size.
+    pub use_pumpfun_v2: bool,
 }
 
 impl SwapParams {