fix(reflow): caret lands on wrong char in centered/right headings

The reflow caret layout hardcoded x0 = columnLeft. That matched only left-aligned
lines; for the new center/right headings the line renders at a shifted x (e.g. a
centered title starts ~215pt while columnLeft is the page margin ~54pt), so the
frontend mapped clicks and drew the caret ~160pt off the actual glyphs.

Record the line's true starting x (after lineX/center/right) as x0 instead. x0 is
caret-only metadata read by the live editor — it does not affect glyph emission or
the rendered page, so output is unchanged (overlay-diff gate identical: 1.70%/1.69%,
STRESS PASS). Rebuilt .pyd + WASM; cache version bumped.

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

engine/src/parser/pdfium_document.cpp

@@ -2982,6 +2982,12 @@ std::expected<void, EngineError> PdfiumDocument::applyEdits(const std::string& e
                         if (align == "right") x = columnLeft + (columnWidth - naturalW);
                         else if (align == "center") x = columnLeft + (columnWidth - naturalW) / 2.0;
                     }
+                    // The line's TRUE starting x (after lineX/center/right). The caret layout below
+                    // records this as x0 so the frontend places the caret on the actual glyphs. (It
+                    // used to hardcode columnLeft, which only matched left-aligned lines — center/right
+                    // headings then mismapped clicks. x0 is caret-only metadata; it does not affect
+                    // glyph emission or the rendered page.)
+                    const double lineStartX = x;
                     for (size_t k = 0; k < lw.size(); ++k) {
                         size_t wi = lw[k];
                         if (k > 0) {
@@ -3026,7 +3032,7 @@ std::expected<void, EngineError> PdfiumDocument::applyEdits(const std::string& e
                         x += words[wi].width;
                     }
                     layoutLines.push_back({
-                        {"baselineY", baselineY}, {"x0", columnLeft},
+                        {"baselineY", baselineY}, {"x0", lineStartX},
                         {"fontSize", lineFontSize > 0 ? lineFontSize : leading / 1.2},
                         {"text", lineText}, {"adv", adv},
                     });