hlkxmerge: add HLKX package merge engine

[elizashurov]

• Add HLKXMerge engine that boots a Studio VM, uploads local HLKX packages, merges them via the HLK API, and downloads the result
• Register hlkxmerge in the engine selector, autoload, and config.json
• Add merge CLI subcommand with --platform, --packages, and --output options

[elizashurov]

@kostyanf14 This PR depends on the rtoolsHCK PR
once it's merged, I'll push a commit bumping rtoolsHCK version

[gemini-code-assist]

Code Review

This pull request introduces a new HLKXMerge engine to merge multiple HLKX packages into one using the HLK API on a Studio VM, registering it across the configuration, autoloading, and CLI options. It also adds safe navigation to prevent a potential nil-pointer exception when iterating over clients. The review feedback highlights two important improvements: first, validating the merge result to ensure failures are not silently ignored and explicitly raising an error; second, validating the platform_name input to prevent a potential path traversal vulnerability when constructing the platform JSON file path.

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