tables: add detectVectorGridInRegion napi export for region-scoped vector grid detection (#70)

* feat: add vector grid region detector napi export

Expose region-scoped vector PDF grid detection so TSR callers can reuse native geometry before model fallback.

Made-with: Cursor

* fix: address vector grid review feedback

Return null for rotated vector grids until the coordinate transform has coverage and reject out-of-crop cell boxes surfaced by real-PDF smoke testing.

Made-with: Cursor

* test: add crop bbox plausibility coverage

Cover in-crop, out-of-crop, slack-boundary, and non-positive DPI behavior for vector grid cell bbox validation.

Made-with: Cursor

napi/test.mjs

@@ -7,6 +7,7 @@ import {
   extractText,
   extractTextWithPositions,
   extractTextInRegions,
+  detectVectorGridInRegion,
   extractPagesMarkdown,
 } from './index.js';
 
@@ -90,6 +91,17 @@ assert.equal(typeof regionResults[0].regions[0].text, 'string');
 assert.equal(typeof regionResults[0].regions[0].needsOcr, 'boolean');
 console.log('  extractTextInRegions: OK');
 
+// --- detectVectorGridInRegion ---
+console.log('Testing detectVectorGridInRegion...');
+const vectorGrid = detectVectorGridInRegion(fixture, 0, [0, 0, 600, 800], 72);
+assert.ok(vectorGrid === null || typeof vectorGrid === 'object');
+if (vectorGrid) {
+  assert.ok(Array.isArray(vectorGrid.structureTokens));
+  assert.ok(Array.isArray(vectorGrid.cellBboxes));
+  assert.ok(vectorGrid.cellBboxes.every(bbox => Array.isArray(bbox) && bbox.length === 4));
+}
+console.log('  detectVectorGridInRegion: OK');
+
 // --- extractPagesMarkdown ---
 console.log('Testing extractPagesMarkdown...');