🔒 Fix capability bypass in web standard library

[Tcode-Motion]

🎯 What: The start, serve, and fetch functions in stdlib/src/web.rs did not verify if the execution context had the Capability::Network permission before initiating network operations, bypassing the runtime's security policy mechanism.
⚠️ Risk: If left unfixed, malicious or sandboxed TechScript code could start an HTTP server or perform HTTP requests even when the network capability is explicitly denied, leading to potential unauthorized network access or SSRF.
🛡️ Solution: Added ctx.config.capabilities.contains(&Capability::Network) checks at the beginning of the respective function callbacks, returning a standard RuntimeError on failure (mirroring the approach in http.rs). Updated register_web to list Capability::Network as a required capability.

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PR created automatically by Jules for task 15282649607834322092 started by @Tcode-Motion

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