perf: tune jemalloc RSS decay for faster memory reclamation (#2428)

The current malloc_conf only sets retain:false, which prevents jemalloc from retaining virtual memory but does not actively purge dirty/muzzy pages. On long-running EasyTier nodes this can lead to inflated RSS because freed arena pages are not returned to the OS promptly.

This tunes the jemalloc decay parameters:

background_thread:true — enables background GC threads so purge happens without blocking allocation hot paths.
dirty_decay_ms:1000 / muzzy_decay_ms:1000 — return unused pages to the OS within ~1s instead of holding them indefinitely.

easytier/src/easytier-core.rs

@@ -14,12 +14,12 @@ static ALLOC: jemallocator::Jemalloc = jemallocator::Jemalloc;
 #[cfg(feature = "jemalloc-prof")]
 #[allow(non_upper_case_globals)]
 #[unsafe(export_name = "malloc_conf")]
-pub static malloc_conf: &[u8] = b"prof:true,prof_active:true,lg_prof_sample:19,retain:false\0";
+pub static malloc_conf: &[u8] = b"prof:true,prof_active:true,lg_prof_sample:19,dirty_decay_ms:1000,muzzy_decay_ms:1000,retain:false\0";
 
 #[cfg(not(feature = "jemalloc-prof"))]
 #[allow(non_upper_case_globals)]
 #[unsafe(export_name = "malloc_conf")]
-pub static malloc_conf: &[u8] = b"retain:false\0";
+pub static malloc_conf: &[u8] = b"dirty_decay_ms:1000,muzzy_decay_ms:1000,retain:false\0";
 
 rust_i18n::i18n!("locales", fallback = "en");