fix(#2050): use stable fnv-1a for avatar idempotency hash

hive-matrix-mcp/src/client.rs

@@ -208,12 +208,18 @@ pub async fn sync_avatar(client: &Client, state_dir: &Path, account: &str) {
             return;
         }
     };
-    // A non-crypto content hash is enough to answer "did the icon change?".
+    // FNV-1a content hash — enough to answer "did the icon change?" and
+    // deterministic across Rust/std versions (unlike `DefaultHasher`, whose
+    // output may change between toolchains, spuriously mismatching the
+    // persisted hash and re-uploading the same avatar). No crypto strength
+    // needed here, so no dependency on a hashing crate.
     let hash = {
-        use std::hash::{Hash, Hasher};
-        let mut h = std::collections::hash_map::DefaultHasher::new();
-        bytes.hash(&mut h);
-        format!("{:016x}", h.finish())
+        let mut h: u64 = 0xcbf2_9ce4_8422_2325;
+        for &b in &bytes {
+            h ^= u64::from(b);
+            h = h.wrapping_mul(0x0000_0100_0000_01b3);
+        }
+        format!("{h:016x}")
     };
     let hash_file = state_dir.join("avatar-icon-hash");
     if let Ok(prev) = fs::read_to_string(&hash_file).await