test(sync-macros): prove public presence expansion

Exercise the proc-macro entry point from a real consumer crate so item preservation and argument routing are mutation-verified; record the clean 33-mutant package gate.

req: sync/003

req: sync/005

req: test/020

req: test/021

hemx-sync-macros/tests/presence.rs

@@ -0,0 +1,62 @@
+extern crate self as hemx_sync;
+
+use hemx_sync_macros::presence;
+
+pub trait IntoEffect {}
+
+impl IntoEffect for &'static str {}
+
+pub trait PresenceScope {
+    fn presence_channel(&self) -> String;
+}
+
+pub trait PresenceUpdate {
+    fn channel(&self) -> &str;
+    fn effect(&self) -> &str;
+}
+
+pub struct PresenceProjection<Effect> {
+    channel: String,
+    effect: Effect,
+}
+
+impl<Effect> PresenceProjection<Effect> {
+    pub fn new(channel: String, effect: Effect) -> Self {
+        Self { channel, effect }
+    }
+}
+
+impl PresenceUpdate for PresenceProjection<&'static str> {
+    fn channel(&self) -> &str {
+        &self.channel
+    }
+
+    fn effect(&self) -> &str {
+        self.effect
+    }
+}
+
+struct Signal(&'static str);
+
+impl PresenceScope for Signal {
+    fn presence_channel(&self) -> String {
+        self.0.to_owned()
+    }
+}
+
+#[presence]
+fn project(signal: Signal) -> impl IntoEffect {
+    if signal.0 == "board" {
+        "joined"
+    } else {
+        "left"
+    }
+}
+
+#[test]
+fn public_presence_attribute_preserves_the_item_and_expands_it() {
+    let update = project(Signal("board"));
+    assert_eq!(update.channel(), "board");
+    assert_eq!(update.effect(), "joined");
+    // test req: sync/003 req: sync/005
+}