refactor(cch): extract BTC_BLOCK_TIME_MILLIS named constant

Replace the magic number `600 * 1000` with a documented named constant
`BTC_BLOCK_TIME_MILLIS` (600,000 ms = 10 minutes per Bitcoin block)
for improved readability and maintainability.

Author: doitian

crates/fiber-lib/src/cch/actor.rs

@@ -32,6 +32,9 @@ use crate::time::{Duration, SystemTime, UNIX_EPOCH};
 pub const ACTION_RETRY_BASE_MILLIS: u64 = 1000; // 1 second initial delay
 pub const ACTION_RETRY_MAX_MILLIS: u64 = 600_000; // 10 minute max delay
 
+/// Average time per Bitcoin block in milliseconds (10 minutes = 600 seconds = 600,000 ms).
+pub const BTC_BLOCK_TIME_MILLIS: u64 = 600_000;
+
 fn calculate_retry_delay(retry_count: u32) -> Duration {
     // Exponential backoff starting from ACTION_RETRY_BASE_MILLIS, capped at ACTION_RETRY_MAX_MILLIS
     let max_shift = (ACTION_RETRY_MAX_MILLIS / ACTION_RETRY_BASE_MILLIS).ilog2();
@@ -515,7 +518,7 @@ impl<S: CchOrderStore> CchState<S> {
         let btc_final_cltv_millis = self
             .config
             .btc_final_tlc_expiry_delta_blocks
-            .checked_mul(600 * 1000)
+            .checked_mul(BTC_BLOCK_TIME_MILLIS)
             .ok_or_else(|| {
                 CchError::ConfigError(format!(
                     "btc_final_tlc_expiry_delta_blocks ({}) is too large and causes overflow when converting to milliseconds",