Improve image safety, stability, and pointer-anchored zoom

[xathay]

Improve image handling safety, resource control, and GUI responsiveness for BigCommunity GNOME.

The previous zoom implementation could briefly move the image away from the pointer before scrolling adjustments caught up. This update synchronizes scale and position and adds a gradual transition.

Changes:

• Isolate executable decoding and metadata inspection in a constrained worker.
• Strengthen input limits, atomic output writes, and bounded batch processing.
• Improve asynchronous loading, previews, and viewer lifecycle cleanup.
• Fix transient zoom jumps and preserve pointer anchoring within scrolling limits.
• Handle fractional touchpad scrolling and respect disabled GTK animations.
• Cancel zoom animations during manual panning, navigation, and window closure.
• Add RAM admission checks for ONNX background removal.
• Fix GNOME icon loading and Nautilus extension packaging.
• Extend regression tests, CI checks, and release documentation.

Local validation:

• 216 Rust tests passed with all features enabled.
• Six graphical zoom scenarios passed, covering 111 painted frames.
• Zoom position error stayed below one screen pixel against the constrained geometric target.
• 40 smoke checks passed on the release binary and another 40 on the packaged binary, covering all 13 supported formats.
• Rustfmt, Clippy, Nautilus contracts, packaging validations, and the offline RustSec audit passed.

Remote CI and the remaining distribution acceptance checks are pending. Full test scope and limitations are documented in docs/QUALITY.md and docs/RELEASE-CANDIDATE.md.