extract_tables: reject font-decode failures via text-density floor (#92)

Some PDFs have fonts whose ToUnicode CMap is missing or broken —
Identity-H fonts without unicode metadata, Type-3 fonts where every
glyph maps to garbage. The page extractor returns punctuation-only
fragments or single-glyph repeats; the rendered image still carries
the visible text, so the region should fall back to OCR rather than
serve a partial table.

The existing captured_only_a_fragment guard can't catch this case
because region_text_chars itself collapses under font-decode failure —
captured vs extracted is symmetrically low, and the ratio still looks
acceptable.

Add a complementary area-based density guard: when a region has lots
of pixel real estate but very few text chars, the page extractor
hit a font failure. Bbox area is independent of extraction success,
so the symmetry breaks.

Threshold 0.003 chars/sq pt sits between observed clean extractions
(≥0.005 on full-page A4 ledgers, key/value layouts, archival
catalogs) and observed font-decode failures (≤0.0014 on prod-traffic
samples). Three guards keep it from misfiring:
  - text_chars < 20 skipped: synthetic / fragmentary fixtures
  - area < 30,000 sq pt skipped: tiny stat blocks
  - area > 400,000 sq pt skipped: near-whole-A4 bboxes where
    density is unreliable (large white-space margins)

Verified against three reproducible cases from prod shadow logs
that previously served partial output:
  - Cyrillic page with punctuation-only decode (46 chars, density
    0.00045) → flagged, routes to OCR
  - Cyrillic page where every glyph collapsed to one letter (89
    chars, density 0.00025) → flagged, routes to OCR
  - Materials-test region where text extracted fine but the table
    body extends beyond the bbox (96 chars, density 0.00131)
    → flagged, routes to OCR

Existing fixtures (full-page A4 ledger, multi-row key/value with
paragraph values, archival catalog, bits_pilani whole-page tests,
synthetic line-grid test) all retain identical behavior.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

napi/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@firecrawl/pdf-inspector",
-  "version": "1.8.13",
+  "version": "1.8.14",
   "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",