Validate page-region citations

Design extraction for the three-page handout-v2 upload in the design exercise: page and block records, the transform for the rotated scan, how the split table stays one table, review rules, and one evaluation measure per stage. Implement validateRegion (JavaScript) or validate_region (Python) to the stated contract. Explain what a passing validation establishes and what it leaves unproven.

Suppose page 1 of a fictional handout was scanned sideways and stored turned 90 degrees clockwise, and OCR ran on the stored image as it is. Coordinates are normalized, with the origin at the upper left of each image.

A point at (u, v) on the upright page appears in the clockwise-turned image at (x', y') = (1 - v, u): the page's top edge becomes the image's right edge. Inverting gives u = y' and v = 1 - x', so a box maps as:

Store the rotation with the page record and apply the inverse before saving evidence. Dividing pixel coordinates by the image size normalizes a box but leaves it in the turned frame. A counterclockwise turn needs a different mapping, derived the same way. Cropping adds an offset and scale, and deskewing a small rotation; undo them in reverse order.

The validator checks one citation region against authoritative document metadata. The caller passes the expected revision and page count from the document record, never from the region itself.

type EvidenceRegion = {
  documentRevision: string;
  pageIndex: number; // zero-based physical page
  box: [number, number, number, number]; // [x0, y0, x1, y1], normalized, upper-left origin
};

// Returns a new { documentRevision, pageIndex, box } with no other fields,
// or throws an Error with the message of the first rule that fails.
declare function validateRegion(
  region: unknown,
  expectedRevision: unknown,
  pageCount: unknown
): EvidenceRegion;

Python names the function validate_region, reads the keys document_revision and page_index, raises ValueError, and returns the four coordinates as floats.

Booleans need an explicit check in both languages: Python's bool is a subclass of int, and JavaScript's Number(true) is 1. The result is built from the checked values, so fields the caller did not ask for never reach downstream code.