Add client-go gap analysis and implementation roadmap

[brendandburns]

Summary

• compare the JavaScript client's capabilities with client-go
• use the Java client as an additional source of cross-language API designs
• prioritize implementation gaps from P0 through P3
• propose phased delivery, explicit non-goals, and success criteria

The analysis emphasizes controller-runtime foundations such as reliable LIST/WATCH behavior, discovery and REST mapping, dynamic clients, informers, workqueues, retries, and test fakes rather than generated API coverage.

Validation

• npx --yes prettier@3.9.6 --check CLIENT_GO_GAP_ANALYSIS.md
• git diff --check

[kubernetes-prow]

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This pull-request has been approved by: brendandburns

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