napi: add probe-doc.mjs for single-region detect_vector_grid checks

Generalized one-shot probe — calls detectVectorGridInRegion against
a single PDF + region without spinning up the api or running the bench.
Useful for fast pdf-inspector dev loops:

  cargo build --release && cd napi && bun run build:debug
  node probe-doc.mjs <pdf-path> [page] [bbox] [dpi]

Output: cells/rows/cols and timing, or null when the region has no
vector grid. ~5s round-trip vs ~30s for the full bench harness.

Generalizes the existing probe-indent.mjs pattern so it works on
arbitrary PDFs, not just the multi-line indent fixture.

napi/probe-doc.mjs

@@ -0,0 +1,64 @@
+#!/usr/bin/env node
+// Generalized one-shot probe: run detectVectorGridInRegion against a single
+// PDF + region without spinning up the api or the bench.
+//
+// Usage:
+//   node probe-doc.mjs <pdf-path> [page] [bbox] [dpi]
+//
+// pdf-path : path to a PDF on disk (required)
+// page     : 0-indexed page number (default 0)
+// bbox     : "x0,y0,x1,y1" in PDF points (default = full standard letter)
+// dpi      : render dpi (default 200)
+//
+// Examples:
+//   node probe-doc.mjs ~/Code/opendataloader-bench/pdfs/01030000000127.pdf 0
+//   node probe-doc.mjs /tmp/foo.pdf 3 "50,100,560,700" 200
+//   node probe-doc.mjs /tmp/foo.pdf       # page 0, full page, dpi 200
+//
+// Prints: cells / rows / cols / null. Useful for fast pdf-inspector loops:
+//   cargo build --release && bun run build:debug && node probe-doc.mjs ...
+
+import { readFileSync, existsSync } from "node:fs";
+import { createRequire } from "node:module";
+
+const require = createRequire(import.meta.url);
+const { detectVectorGridInRegion } = require("./index.js");
+
+const [, , pdfPath, pageArg, bboxArg, dpiArg] = process.argv;
+
+if (!pdfPath) {
+  console.error("usage: node probe-doc.mjs <pdf-path> [page] [bbox] [dpi]");
+  process.exit(2);
+}
+if (!existsSync(pdfPath)) {
+  console.error(`pdf not found: ${pdfPath}`);
+  process.exit(2);
+}
+
+const page = pageArg !== undefined ? Number(pageArg) : 0;
+const bbox = bboxArg
+  ? bboxArg.split(",").map(Number)
+  : [0, 0, 612, 792]; // standard US letter
+const dpi = dpiArg !== undefined ? Number(dpiArg) : 200;
+
+if (bbox.length !== 4 || bbox.some(Number.isNaN)) {
+  console.error(`invalid bbox "${bboxArg}", expected "x0,y0,x1,y1"`);
+  process.exit(2);
+}
+
+const pdf = readFileSync(pdfPath);
+
+const t0 = Date.now();
+const result = detectVectorGridInRegion(pdf, page, bbox, dpi);
+const ms = Date.now() - t0;
+
+if (!result) {
+  console.log(`null (${ms}ms)  page=${page} bbox=${bbox.join(",")} dpi=${dpi}`);
+  process.exit(0);
+}
+
+const rows = result.structureTokens.filter((t) => t === "<tr>").length;
+const cols = rows > 0 ? result.cellBboxes.length / rows : 0;
+console.log(
+  `cells=${result.cellBboxes.length} rows=${rows} cols=${cols} (${ms}ms)  page=${page} bbox=${bbox.join(",")} dpi=${dpi}`,
+);