stream: emit graph trailer distance groups in pure emission order (SPEC 16.1)

Drop the fixed targets,related,extended prefix; iterate the insertion-ordered
group-count dict directly, matching gcf-go/rust/swift/kotlin. Byte-identical for
contract-conformant ascending input; deterministic for distance_N groups. Pinned
by shared fixtures streaming-v2/010-011.

Author: blackwell-systems

CHANGELOG.md

@@ -15,6 +15,7 @@
 
 ### Conformance and docs
 
+- Streaming graph trailer now emits `distance_N` group counts in pure group-header emission order (dropping a fixed `targets,related,extended` prefix), matching the other SDKs and deterministic per SPEC 16.1. Byte-identical for contract-conformant (ascending-distance) input; pinned by shared fixtures `streaming-v2/010`–`011`.
 - The conformance runner now executes the `graph-stream-encode` fixtures (streaming-encode parity, previously decode-only): fixture 004 (positional trailer) and 005 (labeled trailer).
 - README: corrected the streaming example trailer from the defunct `## _summary … sections=` to the real `##! summary … counts=`; README now leads with the project diagram.
 - Added a generic-delta fuzz test (decoder never crashes; string round-trip).

src/gcf/stream.py

@@ -126,16 +126,13 @@ def close(self) -> None:
         with self._lock:
             # Build label:count sections, then either emit as-is (labeled form,
             # SPEC 8.4.1) or strip to values (default positional form).
-            sections: list[str] = []
-            group_order = ["targets", "related", "extended"]
-
-            for g in group_order:
-                c = self._group_counts.get(g, 0)
-                if c > 0:
-                    sections.append(f"{g}:{c}")
-            for g, c in self._group_counts.items():
-                if g not in group_order and c > 0:
-                    sections.append(f"{g}:{c}")
+            # One entry per non-empty distance group in group-header emission order
+            # (SPEC 8.4). The dict preserves insertion order, which is the order the
+            # group headers were emitted, so the trailer is deterministic and matches
+            # the section order (including distance_N groups) across all SDKs.
+            sections: list[str] = [
+                f"{g}:{c}" for g, c in self._group_counts.items() if c > 0
+            ]
             # 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