fix: improve table detection for numeric columns and multi-line headers (#35)

* fix: improve heuristic table detection for numeric columns and multi-line headers

Two fixes for tables that have clean extractable text but fail heuristic
structure detection:

1. Numeric column merge pass (grid.rs): After initial X-position
   clustering, adjacent clusters are merged when one is sparse (header
   text) and the other is dense with >50% numeric items (data column).
   Multi-line wrapped headers often land slightly offset from their
   data column — the merge closes gaps within 1.5× the clustering
   threshold. New is_numeric_text() helper matches decimals, percentages,
   negative numbers, and comma-separated thousands.

2. Duplicate-header skip (detect_heuristic.rs): Spanning super-headers
   like "First Degree | First Degree | Higher Degree" contain duplicate
   cells that trigger looks_like_partial_table_ex rejection. Now skips
   rows with duplicate cells when a better header candidate exists
   within the next 3 rows (higher fill ratio or numeric cells).

Tested on BITS Pilani university report (430 pages, 314 table pages).
Page 4 (multi-line header + numeric data) previously returned
needs_ocr=true; now correctly detects the table structure.

Eval: 197 PDFs, zero regressions, all 104+ tests pass, zero clippy.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* bump version to 0.7.1

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

napi/package.json

@@ -1,6 +1,6 @@
 {
   "name": "firecrawl-pdf-inspector",
-  "version": "0.7.0",
+  "version": "0.7.1",
   "description": "Fast PDF classification and text extraction. Detect text-based vs scanned PDFs, extract text by region with quality checks. Native Rust performance via napi-rs.",
   "main": "index.js",
   "types": "index.d.ts",