perf(thumbnails): SIMD resize via fast_image_resize (PNG 2.6x, RAM 2.5x lower)

Replace the image crate's scalar resampler with fast_image_resize
(AVX2/SSE4.1/NEON) in a shared encode_thumbnail() helper. render_all now
converts to RGB8 once and SIMD-resizes the shared buffer per size. Lanczos3 for
downscaling, CatmullRom when upscaling (Lanczos rings on enlargement).

Also folds the duplicated path-variant generate_all_sizes_background into the
shared render path -- it had missed BOTH shrink-on-load and SIMD resizing -- so
every thumbnail path now goes through one optimised routine (no duplication).

Measured on 14 cores vs the post-1.5 state (benches/BASELINE.md):
- PNG 2.60x faster (33.6->12.9ms), GIF/WebP 1.25-1.6x: full-resolution decode
  paths where the resize dominates, so SIMD helps most
- JPEG only ~7% (shrink-on-load already shrank the bitmap) but peak heap fell
  another ~2.5x (17.6->7.1MB): tight RGB buffers, RGB conversion once
- quality SSIM 0.986-0.994 at identical dims (>=0.98 gate)

Thumbnails are now exactly max_dim on the long side (e.g. 400x266) vs the old
fit-within 399x266 -- a <=1px change, invisible under object-fit: cover.

Bench example gains an exact-dims quality reference + semaphore throughput table.

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

Cargo.toml

@@ -43,6 +43,9 @@ image = { version = "0.25.10", default-features = false, features = ["jpeg", "pn
 # Rust, no C toolchain. The `image` crate's zune-jpeg backend can't scale during
 # decode; this can, cutting decode time/RAM ~order-of-magnitude on large photos.
 jpeg-decoder = "0.3"
+# SIMD (AVX2/SSE4.1/NEON) image resizing for thumbnails — far faster than the
+# `image` crate's scalar resampler, and it speeds every format (incl. PNG/WebP).
+fast_image_resize = "5"
 id3 = "1.17"
 mp3-duration = "0.1"
 kamadak-exif = "0.6.1"