Replace README.md with final peer-reviewed content for MedGemma Impact Challenge

[Copilot]

Full replacement of README.md with the finalized, peer-reviewed technical write-up optimized for Kaggle submission standards.

• Content: Replaces 583-line README with a focused 103-line version covering the 3-layer KVTC architecture, multimodal gatekeeping (PaliGemma × MedGemma), and edge deployment
• Scope: Documentation-only change; no code modified, no tests affected
• Text provided verbatim per issue instructions — wording, formatting, and metrics are locked and not to be altered

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