feat: opt-in lossless (blocking) output for nodes and fanouts

Channels drop on overflow by default. That is the right behaviour for live
sources, where a stale item is worth less than a fresh one, but it makes the
library unusable for offline batch work: items vanish with no diagnostic, and
any downstream analysis that assumes a fixed sample rate is silently invalid.

Auto-inserted fanouts were the harder half of this. They are created inside
make_network(), so user code cannot reach them to configure, and they drop
per-output inside a swallowed catch — so a pipeline whose own nodes were all
configured lossless could still lose items with nothing reported anywhere. In
the pipeline this came from, capture's 2505 frames arrived at the detector as
774 while the overflow counter read zero.

Adds:
  - INode::set_lossless_output(bool), defaulted to a no-op so node types with
    no output channels ignore it
  - PoolNode / PoolObjectNode: route push_one_out() through push_blocking()
  - FanoutNode: same, plus set_lossy_output(i) to opt a single branch back out
  - StaticNetwork::set_lossless(), which reaches user nodes and fanouts alike
  - StaticNetwork::drain(), a public wrapper over the existing private
    drain_all_channels(), so callers can flush in-flight work before stop()

Default behaviour is unchanged; every path is off unless explicitly enabled.

Blocking output is only safe when every consumer eventually drains. A branch
that can stall indefinitely — a display node nobody is servicing — will apply
backpressure to the whole pipeline, which is what set_lossy_output() is for.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

include/kpn/inode.hpp

@@ -37,6 +37,10 @@ struct INode {
     // halt(): alias for stop() — immediate, discards in-flight work.
     virtual void halt() { stop(); }
 
+    // Opt into lossless (blocking) output for nodes that support it.  Default
+    // is a no-op so node types with no output channels ignore it.
+    virtual void set_lossless_output(bool) {}
+
     // shutdown(): graceful drain before stopping. Base implementation falls
     // back to stop(). Network and StaticNetwork override with topo-ordered drain.
     virtual void shutdown() { stop(); }