fix: always emit edge count as last graph trailer entry (SPEC 8.4)

Zero-edge graph streams dropped the trailing edge count (counts=2,1 vs 2,1,0;
labeled omitted edges:0), violating SPEC 8.4/8.4.1. Decoder-ignored trailer;
producer output only. Verified by conformance fixtures 008/009.

Author: blackwell-systems

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## v2.4.0 (2026-07-12)
 
+### Fixes
+
+- Graph streaming trailer: the edge count is now always the last `counts` entry, even when the stream has no edges (positional `counts=2,1,0`; labeled `counts=…,edges:0`). A zero-edge stream previously dropped it, violating the SPEC §8.4 / §8.4.1 rule that the edge count is always present and last (the invariant that keeps the positional form unambiguous). The graph trailer is decoder-ignored, so this changes producer output only.
+
 ### Streaming: opt-in labeled trailer counts (SPEC §8.4.1)
 
 - New `labeled_trailer_counts` keyword on `StreamEncoder`. When set, the `##! summary` graph streaming trailer emits `counts=` in the labeled form `label:count` per group (e.g. `counts=targets:2,related:1,edges:3`) instead of the default positional values-only form (`counts=2,1,3`). Default false is byte-identical to prior output.

src/gcf/stream.py

@@ -136,8 +136,11 @@ def close(self) -> None:
             for g, c in self._group_counts.items():
                 if g not in group_order and c > 0:
                     sections.append(f"{g}:{c}")
-            if self._edge_count > 0:
-                sections.append(f"edges:{self._edge_count}")
+            # The edge count is always the last counts entry, even when 0 (SPEC
+            # 8.4, 8.4.1): it keeps the positional form unambiguous and anchors the
+            # labeled form (minimal counts=edges:0). Zero-count distance groups are
+            # omitted, but edges is not.
+            sections.append(f"edges:{self._edge_count}")
 
             if self._labeled_trailer_counts:
                 counts_str = ",".join(sections)