fix: park nodes on a full output instead of blocking the worker

push_blocking parked a scheduler worker inside the push. Nodes own a
private single-thread pool, so the parked thread was the only one that
could drain that node's own input — hold-and-wait, and under sustained
backpressure four nodes of a five-node chain slept in nanosleep at once.
channel.hpp already warned about this for sentinels; it applies just as
much to data pushes.

The scheduler was purely input-driven: on_input_ready() wakes a node when
input arrives, with no counterpart for "my output has room". Lacking that
signal, blocking the thread was the only way to handle a full output.
This adds the missing half.

- Channel::try_push + has_space + set_space_callback; the callback fires
  from both pop() and try_pop_now().
- PoolNode/PoolObjectNode keep a one-slot pending_ buffer with per-element
  done flags, so a retry cannot duplicate an already-accepted element. One
  slot suffices because queued_ admits at most one fire_once per node.
- The re-check after clearing queued_ closes the lost-wakeup race where a
  space callback fires while the flag is still up and is swallowed.

Two bugs surfaced once nodes actually parked, both fixed here:

- pop_one reports an *empty* channel as ChannelClosedError, which is also
  the node's "upstream finished, self-stop" signal. A node woken by output
  space with empty inputs therefore killed itself. fire_once now releases
  the worker when its inputs are not ready rather than falling through.
- The drained-park path resubmitted unconditionally instead of via
  on_input_ready(), firing nodes with nothing to read.

compute_priority is now output-aware: mean output fill is deducted from
mean input fill, mapped as 0.5·(1 + in - out). Input fill alone asks only
"how much work is waiting for me"; a node whose outputs are already full
cannot deliver, so running it just parks it again and wastes the slot
while the node that would drain that channel waits behind it. The
scheduler now favours whoever is furthest downstream of a bottleneck.

Also adds a network-level error listener. A node's exception was discarded
at the node boundary and survived only as a Closed event, which reports
that a node stopped but not why — that missing detail is what made the
above slow to diagnose. INode::set_network_error_callback plus
StaticNetwork::set_error_handler forward it to the application.

Tests: 121/121. test_backpressure_deadlock drives a five-node chain with
capacity-2 channels against a slow sink and fails on the old code. The
four test_pool_node overflow tests now assert parking rather than the
removed drop-and-report behaviour.

Known-incomplete: a rare hang remains, roughly 1 run in 20 against a 300s
timeout, down from every run failing. Committed because the fix is a large
strict improvement and the residual case needs its own reproduction.

tests/test_backpressure_deadlock.cpp

@@ -0,0 +1,85 @@
+// Regression: a blocking push must not park a pool worker.
+//
+// Node outputs use push_blocking so a full channel costs time rather than data
+// (a dropped frame does not degrade a downstream result, it silently changes
+// one). But push_blocking sleeps *inside* fire_once, which runs on a pool
+// worker — and nodes are pinned to workers by index. Park enough workers in
+// that retry loop and there is nobody left to run the consumer that would drain
+// the channel, so the whole chain wedges.
+//
+// This is the failure channel.hpp:174 already warns about for sentinels
+// ("a blocking push would park that thread and stop it draining its own input,
+// cascading into a hold-and-wait deadlock under backpressure"). The warning
+// applies to data pushes too.
+//
+// Observed in the field as an intermittent hang: frame_source, camera_pos,
+// face_detector and face_aligner all asleep in push_blocking at once.
+#include <catch2/catch_test_macros.hpp>
+#include <kpn/kpn.hpp>
+
+#include <atomic>
+#include <chrono>
+#include <thread>
+
+namespace {
+
+struct Produce {
+    static constexpr std::string_view label() { return "produce"; }
+    int n{0};
+    int operator()() { return n++; }
+};
+
+struct Relay {
+    static constexpr std::string_view label() { return "relay"; }
+    int operator()(int v) { return v; }
+};
+
+// Deliberately slower than the producer, so the channels between them fill.
+struct SlowSink {
+    static constexpr std::string_view label() { return "slow_sink"; }
+    std::atomic<int>* seen;
+    void operator()(int) {
+        std::this_thread::sleep_for(std::chrono::milliseconds(2));
+        seen->fetch_add(1, std::memory_order_relaxed);
+    }
+};
+
+}  // namespace
+
+TEST_CASE("a saturated chain keeps draining", "[backpressure][deadlock]") {
+    std::atomic<int> seen{0};
+
+    Produce  p_fn;
+    Relay    r1_fn, r2_fn, r3_fn;
+    SlowSink s_fn{&seen};
+
+    // Small channels so they saturate immediately, and a chain longer than a
+    // modest pool — the shape that starves workers.
+    kpn::ObjectNode<Produce,  kpn::in<>,        kpn::out<"a">, "produce",   0> p (p_fn,  2);
+    kpn::ObjectNode<Relay,    kpn::in<"a">,     kpn::out<"b">, "relay1",    0> r1(r1_fn, 2);
+    kpn::ObjectNode<Relay,    kpn::in<"b">,     kpn::out<"c">, "relay2",    0> r2(r2_fn, 2);
+    kpn::ObjectNode<Relay,    kpn::in<"c">,     kpn::out<"d">, "relay3",    0> r3(r3_fn, 2);
+    kpn::ObjectNode<SlowSink, kpn::in<"d">,     kpn::out<>,    "slow_sink", 0> s (s_fn,  2);
+
+    auto net = kpn::make_network(
+        kpn::edge(p.output<"a">(),  r1.input<"a">()),
+        kpn::edge(r1.output<"b">(), r2.input<"b">()),
+        kpn::edge(r2.output<"c">(), r3.input<"c">()),
+        kpn::edge(r3.output<"d">(), s.input<"d">())
+    );
+    net.start();
+
+    // The sink is the slowest stage at 2 ms/item, so 40 items is ~80 ms of real
+    // work. Anything approaching the timeout means the chain stopped draining
+    // rather than merely running slowly.
+    const auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(20);
+    while (seen.load(std::memory_order_relaxed) < 40 &&
+           std::chrono::steady_clock::now() < deadline)
+        std::this_thread::sleep_for(std::chrono::milliseconds(5));
+
+    const int got = seen.load(std::memory_order_relaxed);
+    net.stop();
+
+    INFO("items drained: " << got << " of 40");
+    CHECK(got >= 40);
+}