fix: bound and serialize image processing

[VincenzoImp]

Layer 3 of the follow-up stack announced in #25, rebased onto the updated dev now that #28 has landed.

Summary

• enforce conservative encoded-image, page-count, and processing working-set budgets
• validate source dimensions before expensive decoding
• serialize native OpenCV work outside the UI isolate
• dispose codecs and native resources deterministically
• prevent duplicate editor routes and concurrent document mutations
• strengthen repeated native-processing and boundary tests

Verification

• formatter and flutter analyze clean
• native OpenCV, memory-budget, concurrent-route, and image-metadata tests pass
• complete stack: 138 tests pass
• the rebase onto current dev was conflict-free, and the resulting tree is identical to the verified one apart from the 1.3.0 release bump

Remaining follow-up

Rebased onto dev and opened here after this layer merges:

1. Improve scanner accessibility and platform support