fix: try_push must distinguish delivered from discarded

try_push returned bool, and returned *true* for a closed channel — so "the
value arrived" and "the value was thrown away because nobody is listening"
were the same answer.

Every caller was nonetheless correct, because both cases mean "stop trying,
do not park and retry". But nothing above the channel could tell the two
apart: a node counting successful pushes counted discards among them, and the
only record of the loss was the channel's own drop counter, visible solely to
whoever read the diagnostics table.

Now a three-way PushResult { Taken, Full, Closed }, matching the shape
SentinelResult already uses. Behaviour is unchanged at every call site —
each treats Closed the same as Taken, and only Full parks — but the
distinction is now available to anyone who needs it, and a scoped enum means
a future caller cannot silently reintroduce the conflation with `if (push)`.

deliver_one benefits immediately: it no longer reaches its teardown path for
a closed channel, only for one that is still full, so the last-ditch throwing
push it does there to record the loss now records an overflow rather than a
drop the channel had already counted.

tests/test_channel.cpp

@@ -298,3 +298,28 @@ TEST_CASE("try_push_sentinel leaves a refused value untouched", "[channel][senti
     CHECK(ch.try_push_sentinel(third) == Channel<std::string>::SentinelResult::Closed);
     CHECK(third == "eof-3");
 }
+
+// Regression: try_push must distinguish delivered from discarded.
+//
+// It returned bool, and returned *true* for a closed channel — so "the value
+// arrived" and "the value was thrown away because nobody is listening" were the
+// same answer. Every caller was nonetheless correct, because both cases mean
+// "stop trying"; but nothing above the channel could tell the two apart, and a
+// producer counting successful pushes counted discards among them. Only the
+// channel's own drop counter knew, and only if someone read the diagnostics.
+TEST_CASE("try_push distinguishes taken, full and closed", "[channel]") {
+    Channel<int> ch(2);
+    int v = 1;
+
+    CHECK(ch.try_push(v) == Channel<int>::PushResult::Taken);
+    CHECK(ch.try_push(v) == Channel<int>::PushResult::Taken);
+    // Ring is full: the value is untouched and the caller keeps it.
+    CHECK(ch.try_push(v) == Channel<int>::PushResult::Full);
+    CHECK(v == 1);
+
+    ch.disable();
+    const auto drops_before = ch.stats().drops.load();
+    CHECK(ch.try_push(v) == Channel<int>::PushResult::Closed);
+    // Discarded, and recorded as such rather than reported as a delivery.
+    CHECK(ch.stats().drops.load() == drops_before + 1);
+}