Refactor ImageState + add highlight-recovery, fix exposure, smooth hi…

[myyc]

…stogram

Large cleanup branch that rewrites the RAW-processing glue and fixes several rendering issues uncovered along the way.

Cleanup & dedup

• Delete dead code: lib/services/isolate_processor.dart, lib/ffi/raw/raw_processor.dart, and the adjustment math in lib/services/image_processor.dart (nothing called it).
• Consolidate the native RAW build: scripts/build_test_libs.sh, dev.myyc.aks.yaml, and linux/CMakeLists.txt all compile raw_processor_wrapper.c which includes lib/ffi/raw/raw_processor_common.c. Previously there were three drifting copies of the same source.
• Move RawPixelData to its own file; extract applyCropToImage into crop_service.dart; extract generateCurveLookupTable into processors/curve_lut.dart (was copy-pasted 3x).

ImageState refactor

• Slim lib/models/image_state.dart from 546 → ~370 LOC.
• New ImageCacheBundle centralises disposal of the 4 ui.Image refs.
• New ProcessingPipeline owns pixel buffers, timers, processor dispatch; fixes the line-406 unawaited-timer leak.

Highlight recovery

• Hardcode libraw params.highlight = 2 (blend) so the one-channel 255 pile-up (sky / window highlights) is redistributed instead of clipped. Plumbing supports clip/blend/reconstruct via HighlightMode enum; a settings panel can expose the selector later — see CLAUDE.md.
• Compensate blend's systematic darkening with libraw's exposure-correction feature: exp_shift = 2.0 (+1.0 EV) with exp_preser = 1.0 so highlights are compressed rather than re-clipped. Lands blend within -15%/+7% of clip's mean brightness across 5 test fixtures, with zero re-introduced 255-pile-up.

Linear-light exposure

• generateExposureLUT and the Vulkan shader's applyExposure both previously did byte * 2^EV on sRGB-encoded values, massively over- amplifying midtones (at +1 EV, sRGB 128 → 256 clipped instead of ~179).
• Both paths now sRGB-decode → multiply by 2^EV → sRGB-encode. CPU and GPU outputs agree to within 1 LSB on a real 24MP RAW across five EV values (-1, -0.5, 0, +0.5, +1).

Histogram smoothing

• Apply a 5-tap binomial blur twice (σ≈1.4) to the 256-bin counts before painting, matching Lightroom's display. Eliminates visible per-bin quantisation jitter and tapers legitimate spikes at 0/255.
• 2D sampling stride is now sqrt(total/target) instead of total/target, so a 24MP image actually samples ~50k pixels (was ~96).

Tests & lints

• 96 tests green (66 unit + 30 native), up from 23.
• New: curve_lut_test, optimized_processor_test, edit_pipeline_test (JSON roundtrip), image_cache_bundle_test, processing_pipeline_test (line-406 regression), image_state_test, exposure_precision_test (CPU ≡ Vulkan across 5 EVs), fixtures_histogram_test (blend regression guard on 5 real fixtures).
• analysis_options.yaml: excludes generated FFI bindings, enables unused_* lints, cleans up 14 pre-existing dead imports/variables.
• Test fixtures test_image_{1,2,3,4}.{arw,raf} gitignored — they're populated from local photo libraries and tests skip gracefully when absent.

Numbers

• 38 files changed, +544 / -2282 LOC net.