Guard malformed dupes and stale ghosts

[ShiroKSH]

Summary

• Guard ghost setup against invalid head ghost data and failed clientside model creation.
• Keep add-one ghost updates from indexing stale ghost state.
• Return decode errors for malformed dupe tables and invalid JSON conversion input.

Testing

• glualint 1.29.0 on lua/advdupe2/sh_codec.lua and lua/advdupe2/cl_ghost.lua
• git diff --check

[ShiroKSH]

@Astralcircle - thegrb93 approved this PR, and the branch is clean. Could you take the final maintainer pass when you have a chance? I can make any requested changes. Thanks!

[thegrb93]

I didn't fully review or test the code btw. Just looked it over for fatal issues.