v1.4.2

Replace github.com/disintegration/imaging with standard library

(removing long standing CVE with parsing malicious TIFF files)

Dropped the disintegration/imaging dependency by implementing equivalent
thumbnail functionality using golang.org/x/image/draw (which was already
present as a transitive dependency). The two operations previously provided
by imaging — scale-to-fill with center-crop (imaging.Thumbnail) and
JPEG encoding (imaging.Encode) — are now handled by helpers in the
thumbs/imgutils package.

• Added imgutils.Thumbnail — resizes and center-crops using CatmullRom
  resampling (comparable quality to Lanczos).
• Added imgutils.EncodeJPEG — encodes to JPEG with specified quality and
  applies the JFIF APP0 DPI fixup required by Kindle.
• Unexported SetJpegDPI, JpegDPIType, and Dpi* constants since they
  are now only used internally by EncodeJPEG.
• Updated thumbs/kfx and thumbs/mobi readers to use the new helpers.

Fix KFX parser failing on newer KFX files

The KFX entity table parser was too strict and used an outdated symbol
table, causing it to reject valid KFX files with entity IDs above 843.

• Bumped largestKnownSymbol from 834 to 851, adding 17 missing
  YJ_symbols (shapes/geometry: ellipse, rectangle, line, polygon, etc.)
  to match the set known to the fb2cng parser.
• Removed strict FindByID validation from indexTableEntry.readFrom,
  keeping only the bounds check. Entity IDs referencing local document
  symbols beyond the known YJ_symbols range are now accepted, preventing
  future breakage when Amazon extends the format.

Add thumbnail extraction tests

• thumbs/imgutils/thumb_test.go — unit tests for Thumbnail (output
  dimensions across various aspect ratios, invalid inputs, pixel content)
  and EncodeJPEG (valid JPEG output, JFIF APP0 header with 300 DPI).
• thumbs/kfx/reader_test.go — end-to-end test parsing _Test.azw8,
  extracting a thumbnail, and verifying a valid 330×470 JPEG with JFIF
  header is produced.
• thumbs/mobi/reader_test.go — end-to-end test parsing _Test.azw3,
  extracting a thumbnail, and verifying a valid 330×470 JPEG with JFIF
  header is produced.

Full Changelog: v1.4.1...v1.4.2