Add safe local compute and image transforms

[dovvnloading]

Summary

This delivers the final local execution product slices without turning Cortex into an IDE or arbitrary-code runner.

• Adds bounded, decimal-based scratch computation for explicit math requests before generation.
• Adds a normal image transformation flow for PNG, JPEG, and WebP uploads: grayscale, contrast, or brightness, with visible progress, Stop, safe errors, preview, and download.
• Runs both features through short-lived local workers with durable status, timeout, cancellation, recovery, and owner-scoped artifacts.
• Updates the API contracts, frozen-app packaging hooks, tests, README, and the authoritative open-source delivery plan.

Safety and behavior

The scratch tool accepts only a small fixed expression language. It exposes no shell, filesystem, network, package-install, subprocess, or arbitrary Python capability. Image changes use fixed typed recipes and byte-derived inputs rather than user paths or model-generated code. Ordinary chat remains usable if execution is unavailable or disabled.

The local worker is a practical process boundary, not a claim of OS-level sandboxing or arbitrary-code isolation. That distinction is documented in the delivery plan.

Validation

• python -m pytest -q — 345 passed, 1 skipped
• npm.cmd run lint --prefix frontend — passed
• npm.cmd run typecheck --prefix frontend — passed
• npm.cmd test --prefix frontend -- --run — 43 passed
• npm.cmd run build --prefix frontend — passed
• git diff --check — passed

Release note

The source-checkout scope is complete. Before distributing a Windows binary, run the existing package smoke check and a short manual verification of the two local profiles; this is a release check, not another product roadmap stage.