fix: TSR auto-fallback bugs found in review, v1.7.2 (#68)

Three fixes to extract_tables_with_structure_auto_mem (added in
1.7.1) caught by external review:

1. multi_row_in_cell over-triggered on legitimate multi-line cells.
   The previous threshold (item span > 1.3× either smallest cell or
   tallest item height) fires on any cell with 2+ y-separated text
   items — including rowspan>1 cells, wrapped descriptions, and
   superscript/subscript runs. Replaced with two gates:
   - skip cells whose declared rowspan > 1 (intentional multi-line)
   - require an actual whitespace gap (>~half a line height)
     between the bottom of one item and the top of the next, in
     PDF-native y-coordinates. Same-line items with tall glyphs or
     superscripts have negative or near-zero gap; truly separate
     visual rows have gap ≈ leading − line-height.
   FNBO regression test still passes; new test covers a rowspan=2
   cell with two visible text lines and verifies no fallback fires.

2. Heuristic returning empty silently replaced TSR markdown with
   "". The auto wrapper now keeps the TSR markdown when the
   heuristic markdown is empty/whitespace and tags fallback_reason
   with `_heuristic_empty` suffix (e.g.
   `multi_row_in_cell_heuristic_empty`). Worst case we ship the
   same wrong-but-non-empty TSR output we'd have shipped before
   1.7.1; we never replace useful output with literally nothing.

3. One bad input blanked the whole batch. Errors from
   detect_tsr_quality_issue or extract_tables_in_regions_mem now
   stay scoped to the single input — that input falls through to
   raw TSR markdown with a `_error` reason label so callers can
   metric on it. Other inputs in the batch are unaffected.

3 new integration tests:
- test_auto_does_not_fire_on_legit_rowspan_cell
- test_auto_keeps_tsr_markdown_when_heuristic_returns_empty
- test_auto_isolates_per_input_failures

All 6 auto tests + full 123-test suite pass. FNBO local replay
still triggers fallback (phantom_empty_row signal in this run) and
emits correct Shawnee/BVP/Sonoma rows with correct census tracts.

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

napi/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@firecrawl/pdf-inspector",
-  "version": "1.7.1",
+  "version": "1.7.2",
   "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",