🔒 Fix missing FileSystem capability check in CanvasOp::Save

[Tcode-Motion]

🎯 What: Fixed a security vulnerability in the CanvasOp::Save operation where canvas files could be saved to the filesystem without verifying if the executing script had the FileSystem capability granted.
⚠️ Risk: Without this check, a malicious script running in a sandboxed environment without file system permissions could bypass the sandbox and write arbitrary files (like SVG canvas outputs) to the host filesystem.
🛡️ Solution: Added a check using ctx.config.capabilities.contains(&Capability::FileSystem) in the CanvasOp::Save match arm in stdlib/src/canvas.rs before calling std::fs::write(). It returns a RuntimeErrorKind::InvalidOperation error if the capability is missing. Also added Capability::FileSystem to the required_capabilities list in the std.canvas module registry.

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PR created automatically by Jules for task 5075834056834474534 started by @Tcode-Motion

[google-labs-jules]

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