fix: reject orphan attachments in generic decoder (SPEC 16.5)

A .field attachment whose name is neither a ^-marked column of its row nor a
>-containing field name (SPEC 7.4.6.1.4) is now rejected as orphan_attachment,
instead of being silently absorbed as an undeclared extra field. The old
leniency (introduced in the v2.2.1 >-field work, which over-broadly disabled the
v2.1.0 orphan reject) was a lossless-round-trip hole: a stray .field decoded to a
record shape no encoder produces, injecting a field onto the last-parsed row.
The legitimate >-named bare attachment still decodes. Verified against the shared
conformance fixtures (016 now rejects; flatten/016 still round-trips).

Author: blackwell-systems

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Fixes
 
+- Decoder: reject an orphan `.field` attachment (a `.field` whose name is neither a `^`-marked column of its row nor a `>`-containing field name, SPEC 7.4.6.1.4) instead of silently absorbing it as an undeclared extra field. Such a stray attachment previously decoded to a record no encoder produces, silently injecting a field onto the last-parsed row (a lossless round-trip hole); now rejected per SPEC 16.5 (`orphan_attachment`).
 - 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)

src/gcf/decode_generic.py

@@ -521,6 +521,11 @@ def _parse_tabular_body(
         if row_has_id:
             inline_idx = 0
 
+            # Columns that carry a `^` marker cell in this row legitimately expect
+            # a `.field` body. Any other `.field` is an orphan (Section 16.5) unless
+            # its name contains `>` (the flatten-fallback attachment, Section 7.4.6.1.4).
+            expected_att = set(traditional_att_fields) | set(inline_att_fields)
+
             while i < len(lines):
                 a_line = lines[i]
                 a_content: str | None = None
@@ -541,6 +546,14 @@ def _parse_tabular_body(
                     att_name, after_name = _parse_attachment_name(rest)
                     after_name_stripped = after_name.lstrip()
 
+                    # Orphan attachment: a `.field` with no matching `^` cell in this
+                    # row is only legitimate for a `>`-named field (Section 7.4.6.1.4).
+                    # Any other unmatched attachment is rejected rather than silently
+                    # injected as an undeclared extra field, which would decode to a
+                    # record no encoder produces (Section 16.5, lossless round-trip).
+                    if att_name not in expected_att and ">" not in att_name:
+                        raise ValueError(f"orphan_attachment: {att_name}")
+
                     # Prefixed inline data.
                     ifs = inline_schemas.get(att_name)
                     if ifs and not after_name_stripped.startswith("{}") and not after_name_stripped.startswith("["):