feat(pdf): docling-parse migration phase 3.1 — space-box glyph fidelity

Two pdfium glyph quirks broke the sanitizer's merging:
- Generated space glyphs carry a default font that blocked every word↔space
  merge under enforce_same_font. Spaces are now font-neutral (0), and the font
  check treats 0 as a wildcard.
- pdfium gives generated spaces a degenerate *zero-width* loose box at the wrong
  baseline, so corner-distance adjacency failed and lines fragmented into words.
  Drop only zero-width spaces (the inter-word gap then drives merge_with's space
  insertion); spaces with real width are kept so justified double-spaces survive.

Net effect with DOCLING_DP_LINES on: the 2305-pg9 word-interleaving is gone, and
general text improves — multi_page 54→22, normal_4pages 108→82, redp5110
300→256, amt_handbook 16→14. Default path (flag off) unchanged (EXACT 3 intact).

Blocker before default-on: pdfium decomposes fi/ffi ligatures into separate
glyphs whose gaps trip the inserted-space rule (`conf iguration`, `di f f i cult`),
regressing 2305-pg9 to 4. docling-parse keeps the ligature as one cell — next fix
is to recompose them.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: artiz

crates/fleischwolf-pdf/src/dp_lines.rs

@@ -101,7 +101,13 @@ fn applicable(a: &Cell, b: &Cell) -> bool {
     if !a.active || !b.active {
         return false;
     }
-    if a.font != b.font && !is_ligature(&a.text) && !is_ligature(&b.text) {
+    // font 0 = unknown/space (font-neutral); ligatures bridge fonts too.
+    if a.font != 0
+        && b.font != 0
+        && a.font != b.font
+        && !is_ligature(&a.text)
+        && !is_ligature(&b.text)
+    {
         return false;
     }
     a.same_orientation(b)
@@ -186,7 +192,15 @@ pub(crate) fn line_cells(glyphs: &[Glyph], page_h: f32) -> Vec<TextCell> {
             // Use the loose box (uniform font ascent/descent + advance) so adjacent
             // glyphs share a top edge, matching docling-parse's `compute_rect`.
             if !g.ll.is_finite() {
-                return None; // a space pdfium gave no box for; can't place it
+                return None;
+            }
+            // Drop *degenerate* space glyphs (zero-width loose box): pdfium's
+            // generated spaces get a zero-width box at the wrong baseline that
+            // breaks corner-distance adjacency. Without them the inter-word gap
+            // drives `merge_with`'s space insertion. Spaces with a real width are
+            // kept (they carry justified double-space information).
+            if g.ch == ' ' && (g.lr - g.ll).abs() < 0.5 {
+                return None;
             }
             let text = g.ch.to_string();
             let ltr = !is_right_to_left(&text);

crates/fleischwolf-pdf/src/pdfium_backend.rs

@@ -317,7 +317,14 @@ fn glyphs(b: &dyn PdfiumLibraryBindings, tp: FPDF_TEXTPAGE, fetch_font: bool) ->
         if ch == '\r' || ch == '\n' {
             continue;
         }
-        let font = if fetch_font { font_hash(b, tp, i) } else { 0 };
+        // Spaces are font-neutral (0): pdfium's generated spaces carry a default
+        // font that would otherwise block every word↔space merge under
+        // enforce_same_font; docling-parse's spaces inherit the run's font.
+        let font = if fetch_font && !ch.is_whitespace() {
+            font_hash(b, tp, i)
+        } else {
+            0
+        };
         let (mut l, mut r, mut bot, mut top) = (0f64, 0f64, 0f64, 0f64);
         let has_box = b.FPDFText_GetCharBox(tp, i, &mut l, &mut r, &mut bot, &mut top) != 0;
         // Loose box: font ascent/descent + glyph advance, uniform per font/size.