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.lock

@@ -2012,6 +2012,15 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "document-features"
+version = "0.2.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d4b8a88685455ed29a21542a33abd9cb6510b6b129abadabdcef0f4c55bc8f61"
+dependencies = [
+ "litrs",
+]
+
 [[package]]
 name = "dotenvy"
 version = "0.15.7"
@@ -2297,6 +2306,18 @@ dependencies = [
  "serde_json",
 ]
 
+[[package]]
+name = "fast_image_resize"
+version = "5.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fbc7fe45cf92b43817ff62a3723e862b85bd1d06288f63007f7645d1d2f7a060"
+dependencies = [
+ "cfg-if",
+ "document-features",
+ "num-traits",
+ "thiserror 2.0.18",
+]
+
 [[package]]
 name = "fastcdc"
 version = "4.0.1"
@@ -3626,6 +3647,12 @@ version = "0.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "92daf443525c4cce67b150400bc2316076100ce0b3686209eb8cf3c31612e6f0"
 
+[[package]]
+name = "litrs"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "11d3d7f243d5c5a8b9bb5d6dd2b1602c0cb0b9db1621bafc7ed66e35ff9fe092"
+
 [[package]]
 name = "lock_api"
 version = "0.4.14"
@@ -4204,6 +4231,7 @@ dependencies = [
  "dashmap",
  "dotenvy",
  "extism",
+ "fast_image_resize",
  "fastcdc",
  "file-rotate",
  "flate2",