fix(ocr): make recognition deterministic (single-threaded ORT)

ORT's multi-threaded float-reduction order varies across runs, flipping the CTC
argmax on low-confidence characters. This was latent until the sharper
supersampled render pushed a noisy fax (sample_with_rotation_mismatch) to
borderline confidence, making its snapshot non-deterministic (pdf_conformance
75/76 drift). Recognition inputs are tiny per-line crops, so single-threaded ORT
costs negligible throughput and restores determinism — the snapshot baseline is
unchanged (the committed output already matched the deterministic one).

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

Author: artiz

crates/fleischwolf-pdf/src/ocr.rs

@@ -47,9 +47,14 @@ impl OcrModel {
             std::env::var("DOCLING_OCR_REC_ONNX").unwrap_or_else(|_| "models/ocr_rec.onnx".into());
         let dict_path =
             std::env::var("DOCLING_OCR_DICT").unwrap_or_else(|_| "models/ppocr_keys_v1.txt".into());
+        // Single-threaded: ORT's multi-threaded float-reduction order varies
+        // across runs, which flips the CTC argmax on low-confidence characters
+        // (e.g. noisy faxes) and makes the snapshot output non-deterministic. The
+        // recognition inputs are tiny per-line crops, so the throughput cost is
+        // negligible.
         let rec = Session::builder()
             .map_err(|e| format!("ocr: builder: {e}"))?
-            .with_intra_threads(crate::intra_threads())
+            .with_intra_threads(1)
             .map_err(|e| format!("ocr: intra_threads: {e}"))?
             .commit_from_file(&rec_path)
             .map_err(|e| format!("ocr: load {rec_path}: {e}"))?;