perf(export): accelerate lossless JXL encoding

[VailElla]

Description

Improve DNG-to-JPEG XL export throughput while preserving lossless output and the existing export validation/metadata/cancellation behavior. Lossless JXL now honors the selected Fast/Balanced/Best effort instead of always using the slowest effort, uses the shared Rayon pool for codec work, parallelizes 16-bit quantization, and avoids redundant 8/16-bit conversions.

This branch is intentionally cumulative: it contains the prerequisite RAW/JXL export stack plus this performance change, so it should be reviewed as the next stacked export update.

Type of Change

• Bug fix
• New feature
• Breaking change
• Performance improvement
• Code refactoring
• Documentation update
• UI/UX improvement
• Build/CI or Dependency update

Changes Made

• Map the lossless JXL encoding-speed selector to efforts 4 (Fast), 5 (Balanced), and 7 (Best compression).
• Enable shared-pool multithreading with with_threads(0) and parallelize high-precision sample quantization.
• Reuse native 8-bit/16-bit image buffers when already in the requested format.
• Keep the export controls and validation/round-trip coverage from the prerequisite JXL stack.

Screenshots/Videos

Manual export was exercised in the local macOS app. The generated JXL files remain local in /tmp/rapidraw-jxl-manual-20260713 and are not uploaded here.

Testing

• I have tested these changes locally and confirmed that they work as expected without issues

Test Configuration:

• OS: macOS 26.5.2
• Hardware: Apple Silicon Mac16,7, 14 CPU cores
• Manual DNG: IMG_2094.DNG, JPEG XL quality 100, 8-bit, metadata included, GPS removal enabled

Observed wall-clock time from pressing Save until the app returned to the export-ready state:

Mode	Time	Output
Fast	~7.8 s	6,272,665 bytes
Balanced	~8.0 s	6,228,328 bytes
Best compression	~43.0 s	3,728,944 bytes

All three outputs were recognized by macOS as JPEG XL containers.

Automated checks:

• cargo +1.96.1 fmt --manifest-path src-tauri/Cargo.toml --check — passed
• cargo +1.96.1 check --manifest-path src-tauri/Cargo.toml — passed (17 pre-existing warnings)
• cargo +1.96.1 test --manifest-path src-tauri/Cargo.toml -- --nocapture — passed (37 tests)
• npm run build — passed
• ./node_modules/.bin/tauri build --debug --bundles app --no-sign — passed
• git diff --check — passed

Known repository baseline checks (unchanged by this branch):

• npm run typecheck currently reports 117 existing TypeScript errors across the repository.
• npm run i18n:check currently reports existing plural-key drift in 11 locale files.

Checklist

• My code follows the project's code style
• I haven't added unnecessary AI-generated code comments
• My changes generate no new warnings or errors (baseline warnings/errors are documented above)

Additional Notes

• The local debug app bundle was built unsigned for manual verification.
• The branch includes the prerequisite export work associated with PR #1365 and PR #1366; the cumulative diff against main is expected.

AI Disclaimer:

• This PR is entirely AI-generated
• This PR is AI-generated but guided by a human
• This PR was handwritten with AI assistance (spell check, logic suggestions, error resolving)
• This PR contains only blood, sweat, and coffee (AI-free)