cold_pack: filter _content_size in single-shard restore — align with pull_metadata_pack NULL contract

_restore_generic_table forwarded every JSONL column to the INSERT
statement, including the producer's synthetic ``_content_size``
column (#53 — emitted on chunks to carry the size hint while the
actual content BLOB is dropped from the metadata pack to keep it
small). The target chunks table has no such column, so the restore
hard-errored on ``no column named _content_size`` before any row
landed; four cold_object tests had been failing on the branch
(push_pack_then_pull_full / just_enough_hydrate / splits_to_fit_dvdr
/ v2_hydrates_fresh_empty_db).

Drop ``_content_size`` from the column list. chunks rows land with
``content IS NULL`` per ``pull_metadata_pack``'s documented contract
(``evict.py:825``); phase 2 (``pull_chunk_pack``) fills bytes via
UPDATE WHERE leaf_hash=?.

The size hint is unused in the single-shard path. The multi-shard
``_restore_routed_table`` still pre-allocates a zero-byte BLOB of
that size for in-place phase 2 UPDATE (avoids page splits at corpus
scale); that's a latent inconsistency with the doc contract on the
just-enough routed path, but no test exercises it and the
performance argument is real at scale. Not touching it here.

Suite: 2995 passed (up from 2991), no regressions.

arborist/cold_pack_metadata.py

@@ -470,7 +470,19 @@ def _restore_generic_table(
     table: str,
     in_path: Path,
 ) -> int:
-    """Read array-per-line JSONL → INSERT OR IGNORE batches into `table`."""
+    """Read array-per-line JSONL → INSERT OR IGNORE batches into `table`.
+
+    Chunks special case (#53): the producer dumps a synthetic
+    ``_content_size`` column carrying each chunk's content byte-length
+    (the actual ``content`` BLOB is dropped from the metadata pack to
+    keep it small). Consumer drops the synthetic column from the INSERT
+    column list — chunks rows land with ``content IS NULL`` per
+    ``pull_metadata_pack``'s documented contract; phase 2
+    (``pull_chunk_pack``) fills the bytes via UPDATE WHERE leaf_hash=?.
+    The size hint is currently unused in the single-shard path; a
+    future zeroblob pre-allocation could read it back if page-split
+    cost becomes measurable.
+    """
     BATCH = 5000
     batch: list[tuple] = []
     cols: list[str] | None = None
@@ -478,7 +490,7 @@ def _restore_generic_table(
     insert_sql: str | None = None
     for row in read_columnar_jsonl(in_path):
         if cols is None:
-            cols = list(row.keys())
+            cols = [c for c in row.keys() if c != "_content_size"]
             placeholders = ", ".join("?" for _ in cols)
             col_list = ", ".join(f'"{c}"' for c in cols)
             insert_sql = (