Support multi-page TIFF preview IFD chains

Add support for TIFF preview/preview-page IFD chains by introducing a page-oriented IFD slot and plumbing it through parsing, building, and time-patch logic.

Key changes:
- README wording clarified to describe bounded preview-page chain rewrite and preserved downstream tails.
- Replaced Ifd1 with a generic page slot (ExifIfdSlot::IfdPage) and extended TransferExifIfdRef with page_index and is_page.
- Added parse_ifd_page_index_name to parse numbered IFD names (ifdN).  Classified page refs during metadata classification.
- Store and handle page IFDs in vectors (page_ifds) instead of a single ifd1.  Implemented resizing, sorting, entry serialization, value placement, and time-patch mapping for page IFDs.
- Parsing: follow and validate preview IFD chains, prevent loops and excessive length, and populate parsed_exif.page_ifds.  Preserve extra existing page IFDs when merging.
- Writing: assemble page chains, set next offsets correctly, and ensure first_page_ifd_off is used as the rewritten next-ifd offset from IFD0.
- Updated endian conversion and parsing/clearing logic to handle multiple page_ifds.
- Tests: added helpers to construct minimal three-page TIFF/BigTIFF fixtures and multiple tests validating multi-page chain round-trip, tail preservation, and updated expectations (use >= offsets).  Adjusted existing tests to assert chain behavior.

These changes enable handling of multiple preview pages (ifdN) in EXIF/TIFF transfers, preserving existing downstream page tails and ensuring correct serialization/deserialization.

Author: ssh4net

README.md

@@ -92,13 +92,13 @@ Current target status:
 
 In practice:
 - JPEG and TIFF are the strongest transfer targets today.
-- TIFF edit support now covers classic TIFF, BigTIFF, bounded `ifd1`
-  chain rewrite with preserved downstream page tails, and bounded
+- TIFF edit support now covers classic TIFF, BigTIFF, bounded preview-page
+  chain rewrite (`ifd1`, `ifd2`, and preserved downstream tails), and bounded
   TIFF/DNG-style SubIFD rewrite with preserved downstream auxiliary tails
   and preserved trailing existing children when only the front subset is
   replaced.
 - For DNG-like TIFF sources, the current bounded merge policy is:
-  replace the source-supplied front preview/aux structures, preserve
+  replace the source-supplied front preview-page/aux front structures, preserve
   existing target page tails and trailing auxiliary children.
 - PNG, WebP, JP2, JXL, bounded BMFF, and EXR all have real first-class
   transfer entry points.