feat: per-page OCR routing reasons (#139)

* feat: per-page OCR routing reasons (scanned/no_text/vector_text/garbled)

Replaces the single suspected_garbled_text signal with a per-page
explanation for why each OCR-flagged page needs OCR. The detector
classifies each page in pages_needing_ocr from its content analysis:

- scanned            — no usable text, image-backed page
- no_text            — no text and no image (blank/unreachable)
- vector_text        — text drawn as vector outlines, not extractable
- suspected_garbled_text — undecodable Identity-H/Type3 fonts

Exposed on PdfTypeResult.ocr_reasons_by_page and surfaced through
PdfProcessResult and the detect-pdf CLI (JSON + human output). Reasons
only ever explain pages already flagged for OCR — a text page with an
embedded logo stays TextBased, so this doesn't widen the OCR net.
Markdown output is byte-identical across the regression corpus.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* review: cache freshly-analyzed pages so OCR reasons aren't lost

Under a sampling ScanStrategy, the Mixed per-page loop (Phase 2) and the
garbled-font check (Phase 3) analyze non-sampled pages but dropped the
PageAnalysis after flagging them. The reason-classification pass then
missed the cache and defaulted those pages to "scanned", masking the
real vector_text / suspected_garbled_text cause. Insert the fresh
analyses into analysis_cache so the reason pass classifies them
correctly. No change under the default full-sampling strategy (all
pages are already cached).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/integration_tests.rs

@@ -1102,6 +1102,7 @@ fn test_pages_needing_ocr_field_accessible() {
         title: None,
         ocr_recommended: false,
         pages_needing_ocr: Vec::new(),
+        ocr_reasons_by_page: std::collections::BTreeMap::new(),
     };
     assert!(detection_result.pages_needing_ocr.is_empty());