fix dead code appearing after panic

[cce]

This fixes Go vet warnings on 1.25 about code appearing after panic() calls.

Diffs of msgp_gen.go in algorand/go-algorand#6468 show the effect.

[Copilot]

Pull Request Overview

This PR addresses Go vet warnings in version 1.25 by preventing dead code from appearing after panic() calls in generated code. The solution introduces a halt mechanism to track when a panic statement has been generated and prevents subsequent code generation.

• Introduces a halted boolean flag to track when panic has been called
• Adds a new panicf() helper method that sets the halt flag
• Updates all code generation methods to check and respect the halt state

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