perf(downloader): 连接池空闲上限提升到 64 匹配多核分段数

旧值 16 按 4 核机器设定，在 16+ 逻辑核主机上下载大文件时（分段数达 64）会饿死空闲连接池，导致第 16 段之后的每个分段都要重新 TCP+TLS 握手。HLS 并发仍独立保持 16 以约束 CDN 单 IP 连接压力。

Author: zerx-lab

native/engine/src/downloader.rs

@@ -650,13 +650,17 @@ pub fn build_client(
         .connect_timeout(Duration::from_secs(15))
         // No global timeout — downloads can be very long
         // Connection pool — keep enough idle connections to cover all
-        // segments of a typical multi-segment download.  16 matches
-        // MAX_SEGMENTS on a 4-core machine (cpu_cores * 4) and avoids
-        // expensive TCP+TLS re-handshakes when workers finish one segment
-        // and immediately start the next.  90 s idle timeout tolerates
-        // brief pauses / UI interaction without discarding warm connections.
+        // segments of a multi-segment download so workers reuse warm
+        // keep-alive connections instead of paying TCP+TLS re-handshake
+        // costs when finishing one segment and starting the next.
+        // 64 == MAX_SEGMENTS (segment_advisor caps io_cap at cpu_cores*4,
+        // which reaches 64 on 16+ logical-core machines downloading large
+        // files).  The previous value of 16 (sized for a 4-core machine)
+        // starved the idle pool on many-core hosts, forcing re-handshakes
+        // for every segment beyond the 16th.  90 s idle timeout reclaims
+        // the extra connections shortly after the download finishes.
         .pool_idle_timeout(Duration::from_secs(90))
-        .pool_max_idle_per_host(16)
+        .pool_max_idle_per_host(64)
         // Cookies — needed for session-based downloads (Google Drive, etc.).
         // reqwest follows RFC 6265: cookies are scoped to their domain.
         .cookie_store(true)

native/engine/src/hls_downloader.rs

@@ -71,10 +71,12 @@ const RETRY_BASE_DELAY: std::time::Duration = std::time::Duration::from_secs(2);
 
 /// Upper bound on concurrent segment downloads.
 ///
-/// Matches `build_client`'s `pool_max_idle_per_host(16)` in `downloader.rs`:
-/// going past the connection-pool size forces expensive TCP+TLS re-handshakes
-/// and risks tripping CDN per-IP connection limits. 16 is the same ceiling the
-/// multi-segment HTTP downloader uses.
+/// Capped at 16 to bound CDN per-IP connection pressure: HLS playlists often
+/// have hundreds of tiny segments, and opening dozens of parallel connections
+/// to a single streaming CDN risks tripping per-IP limits or throttling.
+/// This ceiling is intentionally independent of `build_client`'s idle-pool
+/// size (`pool_max_idle_per_host`, sized for the 64-segment HTTP path) —
+/// the pool is large enough to keep every HLS connection warm regardless.
 const MAX_HLS_CONCURRENCY: usize = 16;
 
 /// Concurrency used when the user left the segment count on "auto"