Add specialized media model routing demo

[nachiketb-nvidia]

What changed

• add a ModelAsTool implementation of libsy's Algorithm trait
• inject generate_media(prompt) into the primary model request
• route selected calls to an image-only local Cosmos client
• expose the algorithm through server TOML and the libsy Python API
• add an A/B config: switchyard/frontier is plain Opus 4.8; switchyard/media is Opus 4.8 plus Cosmos

Validation

• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• pre-commit hooks passed
• live Pi passthrough returned FRONTIER_PASSTHROUGH_OK
• live Pi media route generated a valid 1024x1024 RGB PNG

[github-actions]

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