perf(thumbnails): shrink-on-load JPEG decode (1.8-2× faster, 5-15× less RAM)

Decode JPEGs at the smallest DCT scale (1/8·1/4·1/2·1/1) whose long axis is
still ≥ the largest needed thumbnail (800px), via jpeg-decoder, instead of a
full-resolution decode through the image crate. The full-res bitmap — the
dominant time and RAM cost — is never materialised. PNG/GIF/WebP and unusual
JPEG colour spaces (CMYK / 16-bit grey) fall back to a full decode.

Extracts the shared decode + EXIF-orientation logic into decode_oriented(),
removing the duplication that existed between render_thumbnail_from_data and
render_all_thumbnails_from_data.

Measured on 14 cores (see benches/BASELINE.md):
- render_all 1.8-2.0× faster (12MP 111->61ms, 48MP 398->203ms)
- peak heap 5.5-14.8× lower, now decoupled from source MP (~18-25MB regardless)
- saturated throughput 3-3.6× (parallel efficiency 4.9×->8.5×)
- quality SSIM 0.987-0.999 (>=0.98 gate), PSNR 47-55dB

Also adds the Phase 0 benchmark harness (gated behind the `bench` feature, zero
prod impact): deterministic image corpus (src/bench_support.rs), criterion
latency bench (benches/thumbnails.rs), and a peak-RAM/throughput/SSIM harness
(examples/bench_thumbnails_mem.rs). Baseline + before/after in benches/BASELINE.md.

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

src/lib.rs

@@ -12,6 +12,12 @@ pub mod interfaces;
 #[cfg(integration_tests)]
 pub mod integration_test_support;
 
+// Phase 0 perf-benchmark support: deterministic image corpus generation/loading
+// shared by `benches/thumbnails.rs` and `examples/bench_thumbnails_mem.rs`.
+// Gated behind the `bench` feature so it adds nothing to normal builds.
+#[cfg(feature = "bench")]
+pub mod bench_support;
+
 // Common public re-exports
 pub use application::services::folder_service::FolderService;
 pub use application::services::i18n_application_service::I18nApplicationService;