feat(r2): C1 ordinals and C2 the review ring and .seen

- Item.ordinal: the 1-based position in booth_items over the items that
  render. It is appended, and set in the resolver. Tiles print it padded to
  the whole set's width, and a filter never renumbers.
- review_chain: the item order filtered to media. It replaces image_chain as
  the zoom route's ring, so a set of pictures and sound steps through both.
  image_chain stays importable.
- .seen: which media items were looked at full size, written by the review
  route under record_view's gate. It is rewritten whole: deduplicated, pruned
  to live items, sorted. The temp file is created with O_EXCL and swapped in
  with os.replace, so a planted symlink is replaced, never written through.
  It never raises.

Nine new tests. The contiguity and symlink tests are mutation-checked.
669 passed.

booth/items.py

@@ -95,6 +95,27 @@ class Item:
     blurred: bool
     doc: str | None
     size: int
+    # R2 C1: the 1-based position in `booth_items` order over ALL items — the
+    # number the operator means by "the third one". Set in the resolver loop
+    # and nowhere else (INV-1). APPENDED, never inserted: a mid-dataclass field
+    # is a positional-construction break.
+    ordinal: int
+
+
+# R2 C2: which items have been looked at full size. UI state, not judgment —
+# never exposed to sessions, holds nothing. One viewer: this records WHAT was
+# seen, never who saw it.
+SEEN_FILE = ".seen"
+
+
+def read_seen(booth: Path) -> set[str]:
+    """Rels seen at full size. Missing or unreadable file -> empty set; a
+    damaged marker costs the tape its memory, never the page."""
+    try:
+        text = (booth / SEEN_FILE).read_text()
+    except (OSError, UnicodeDecodeError):
+        return set()
+    return {ln.strip() for ln in text.splitlines() if ln.strip()}
 
 
 def read_blurred(booth: Path) -> set[str]:
@@ -273,6 +294,10 @@ def booth_items(booth: Path) -> list[Item]:
                 blurred=rel in blurred,
                 doc=doc_kind(p.name),
                 size=size,
+                # Counted over items that RENDER: a caption sidecar or a name
+                # the quote() guard skipped takes no number, so the numbers
+                # stay contiguous over what the operator can see.
+                ordinal=len(items) + 1,
             )
         )
     return items
@@ -287,6 +312,18 @@ def image_chain(items: Sequence[Item]) -> list[str]:
     return [it.rel for it in items if it.kind == "image"]
 
 
+# R2 C2: what the review route steps through. ONE LINE: the item order
+# filtered to media. It is a declared change to the zoom-ring rule, which was
+# images only: a listening set is reviewed the same way a picture set is.
+REVIEW_KINDS = ("image", "video", "audio")
+
+
+def review_chain(items: Sequence[Item]) -> list[str]:
+    """The rels of the media items, in item order — the review's prev/next ring,
+    its filmstrip and its tape."""
+    return [it.rel for it in items if it.kind in REVIEW_KINDS]
+
+
 def find_item(items: Sequence[Item], rel: str) -> Item | None:
     """The record for one rel, or None — the zoom/doc route's entry point."""
     for it in items: