fix(pump): restore 26-byte legacy buy instruction data encoding

The refactored ix data encoder was producing 25-byte buy instructions
(1-byte track_volume), but the on-chain Pump program expects Anchor's
2-byte Option<bool> encoding (option tag + value), totaling 26 bytes.
The 1-byte shortfall caused instruction data under-read that manifested
as error 0xbbf (ConstraintOwner) for all legacy buy/buy_exact_sol_in.

Restore the 2-byte encoding: option tag (1u8) followed by the cashback
flag as the bool value, matching the pre-refactoring inline encoder.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/instruction/pumpfun.rs

@@ -12,7 +12,7 @@ use crate::{
     instruction::pumpfun_ix_data::{
         encode_pumpfun_buy_exact_quote_in_v2_ix_data, encode_pumpfun_buy_exact_sol_in_ix_data,
         encode_pumpfun_buy_ix_data, encode_pumpfun_buy_v2_ix_data,
-        encode_pumpfun_sell_ix_data, encode_pumpfun_sell_v2_ix_data, TRACK_VOLUME_TRUE,
+        encode_pumpfun_sell_ix_data, encode_pumpfun_sell_v2_ix_data,
     },
     instruction::utils::pumpfun::{
         accounts, get_bonding_curve_pda, get_bonding_curve_v2_pda,
@@ -230,15 +230,16 @@ impl InstructionBuilder for PumpFunInstructionBuilder {
         }
 
         // ── Legacy buy path ──
+        let track_volume_val = if bonding_curve.is_cashback_coin { 1u8 } else { 0u8 };
         let buy_data = if params.use_exact_sol_amount.unwrap_or(true) {
             let min_tokens_out = calculate_with_slippage_sell(buy_token_amount, slippage_bp);
             encode_pumpfun_buy_exact_sol_in_ix_data(
                 lamports_in,
                 min_tokens_out,
-                TRACK_VOLUME_TRUE,
+                track_volume_val,
             )
         } else {
-            encode_pumpfun_buy_ix_data(buy_token_amount, max_sol_cost, TRACK_VOLUME_TRUE)
+            encode_pumpfun_buy_ix_data(buy_token_amount, max_sol_cost, track_volume_val)
         };
 
         let bonding_curve_v2 = get_bonding_curve_v2_pda(&params.output_mint).ok_or_else(|| {

src/instruction/pumpfun_ix_data.rs

@@ -7,20 +7,22 @@ use crate::instruction::utils::pumpfun::{
     BUY_V2_DISCRIMINATOR, SELL_DISCRIMINATOR, SELL_V2_DISCRIMINATOR,
 };
 
-/// 与官方 `getBuyInstructionInternal` 一致：`track_volume = true`。
-pub const TRACK_VOLUME_TRUE: u8 = 1;
+/// 与官方 `getBuyInstructionInternal` 对齐：`OptionBool` 在 ix 中为 1 字节 option tag + 1 字节值。
+/// `track_volume` 在调用侧组合为 `(1u8, cashback as u8)`（Option tag + value），共 26 字节 ix data。
+pub const TRACK_VOLUME_OPTION_TAG: u8 = 1;
 
 #[inline(always)]
 pub fn encode_pumpfun_buy_ix_data(
     token_amount: u64,
     max_sol_cost: u64,
-    track_volume: u8,
-) -> [u8; 25] {
-    let mut d = [0u8; 25];
+    track_volume_val: u8,
+) -> [u8; 26] {
+    let mut d = [0u8; 26];
     d[..8].copy_from_slice(&BUY_DISCRIMINATOR);
     d[8..16].copy_from_slice(&token_amount.to_le_bytes());
     d[16..24].copy_from_slice(&max_sol_cost.to_le_bytes());
-    d[24] = track_volume;
+    d[24] = TRACK_VOLUME_OPTION_TAG;
+    d[25] = track_volume_val;
     d
 }
 
@@ -28,13 +30,14 @@ pub fn encode_pumpfun_buy_ix_data(
 pub fn encode_pumpfun_buy_exact_sol_in_ix_data(
     spendable_sol_in: u64,
     min_tokens_out: u64,
-    track_volume: u8,
-) -> [u8; 25] {
-    let mut d = [0u8; 25];
+    track_volume_val: u8,
+) -> [u8; 26] {
+    let mut d = [0u8; 26];
     d[..8].copy_from_slice(&BUY_EXACT_SOL_IN_DISCRIMINATOR);
     d[8..16].copy_from_slice(&spendable_sol_in.to_le_bytes());
     d[16..24].copy_from_slice(&min_tokens_out.to_le_bytes());
-    d[24] = track_volume;
+    d[24] = TRACK_VOLUME_OPTION_TAG;
+    d[25] = track_volume_val;
     d
 }