docs: drop AI/ML terminology in README, document marker ordering

- README "How it works" step 3: "not by any AI/ML step" -> "not by any
  model inference step", per AGENTS.md's terminology rule (no generic
  "AI" when describing system capability).
- AppConfig gets a docstring spelling out that markers must be ordered
  smallest-bucket-boundary first, since determine_size_bucket trusts
  this order and a misordered config would silently produce a wrong
  (tax-relevant) size bucket. AppConfig.load() also now validates that
  marker area is non-decreasing across the list and raises ValueError
  if not, since this project's nested physical markers get larger as
  bucket size increases.

README.md

@@ -30,8 +30,9 @@ volunteer/client work — it isn't part of any commercial product line.
 2. A volunteer centers the book; capture fires automatically or on a
    button press (configurable).
 3. The system photographs the front, determines size — **deterministically**,
-   by checking which physical markers are covered, not by any AI/ML step —
-   then prompts the volunteer to flip the book and photographs the back.
+   by checking which physical markers are covered, not by any model
+   inference step — then prompts the volunteer to flip the book and
+   photographs the back.
 4. Every book gets a row in a CSV catalogue: images, size bucket, and a
    status that later pipeline stages (OCR backfill, checkout) update.
 

bookmark/config.py

@@ -7,6 +7,23 @@ from .sizing import MarkerRegion
 
 @dataclass
 class AppConfig:
+    """
+    IMPORTANT — `markers` ordering requirement:
+
+    `markers` must be listed smallest-bucket-boundary first, largest last
+    (see `bookmark.sizing.determine_size_bucket`, which trusts this order
+    and does not re-sort). The physical rig's nested markers get larger as
+    the size bucket grows, so marker area (width * height) should be
+    non-decreasing across the list in config.json. `AppConfig.load()`
+    validates this area ordering and raises `ValueError` if it's violated,
+    but it cannot verify that markers are ordered by *bucket name*
+    ("small" < "medium" < "large") independent of that, since bucket
+    naming is config-defined — an install with correctly-increasing marker
+    areas but mislabeled bucket names would still silently produce a wrong
+    (but plausible-looking) size bucket. Get the marker order right in
+    config.json; this is tax-report-relevant data for Books in Hand.
+    """
+
     device_index: int
     catalogue_path: Path
     captures_dir: Path
@@ -19,6 +36,7 @@ class AppConfig:
     def load(cls, path: Path) -> "AppConfig":
         data = json.loads(Path(path).read_text())
         markers = [MarkerRegion(**marker) for marker in data["markers"]]
+        _validate_marker_area_order(markers)
         return cls(
             device_index=data["device_index"],
             catalogue_path=Path(data["catalogue_path"]),
@@ -28,3 +46,24 @@ class AppConfig:
             guide_box=tuple(data["guide_box"]),
             trigger_strategy=data["trigger_strategy"],
         )
+
+
+def _validate_marker_area_order(markers: list[MarkerRegion]) -> None:
+    """
+    `determine_size_bucket` requires `markers` ordered smallest -> largest
+    bucket boundary. We can't infer bucket order from names alone, but the
+    physical rig's nested markers get larger as the bucket size grows, so
+    marker area (width * height) should be non-decreasing across the
+    ordered list. A misordered config would silently produce a wrong (but
+    plausible-looking) tax-relevant size bucket, so fail loudly here
+    instead.
+    """
+    areas = [marker.width * marker.height for marker in markers]
+    for previous, current in zip(areas, areas[1:]):
+        if current < previous:
+            raise ValueError(
+                "config.json 'markers' must be ordered smallest-bucket to "
+                "largest-bucket, with non-decreasing marker area — found a "
+                f"marker with area {current} listed after one with area "
+                f"{previous}. Check the marker order in config.json."
+            )