fix(design): isolate screenshot Chromium via egress proxy

[echobt]

Summary

• Force screenshot Chromium (--no-sandbox, file://) through design-egress-proxy with --proxy-bypass-list=<-loopback> so control-plane / metadata SSRF is blocked by the same post-DNS blocklist as miner sandboxes.
• Neutralize SSRF-ish href/src (gateway, socket-proxy, postgres, RFC1918, metadata) in ammonia sanitize; capture documents get nonce-locked CSP + navigate-to 'none' (CLI Chromium has no Playwright route hooks).
• Compose sets DESIGN_SCREENSHOT_PROXY; assert-compose-matrix confirms staging/prod keep host Sim fail-closed and force the proxy. Produced HTML remains unserved.

Test plan

• cargo test -p design-sanitize -p design-challenge --lib
• cargo clippy -p design-sanitize -p design-challenge --all-targets -- -D warnings
• ./deploy/scripts/assert-compose-matrix.sh
• cargo run -p xtask -- design-check
• Staging deploy recreates design-challenge with DESIGN_SCREENSHOT_PROXY=http://design-egress-proxy:8094
• Spot-check a new design run still gets index.png

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Configuration used: defaults

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📥 Commits

Reviewing files that changed from the base of the PR and between 1e688ad and 9ffdff9.

📒 Files selected for processing (7)

• crates/design-challenge/src/screenshot.rs
• crates/design-sanitize/src/lib.rs
• deploy/scripts/assert-compose-matrix.sh
• docker-compose.yml
• docs/DESIGN_CHALLENGE.md
• docs/OPERATOR_SECURITY.md
• docs/THREAT_MODEL.md

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