feat(sprites): ✨ OSS standin coverage p2-23..27 (536 PNGs) + xi-v11 charter
- 536 game-icons.net CC-BY-3.0 standins fill every renderer slot (units/buildings/wonders/city-tiers), id-keyed flat layout
- LICENSES.md (536 ledgered rows, SHA256), STANDINS.md, sprite-license-audit passes
- build_standins.py rewritten data-driven off manifest + icon_rules.json (replaces mapping.json)
- juggernaut-xi-v11 added to approved model list (charter + 2 instruction modules), operator decision
- objectives p2-23..27 + p2-22: partial (standin coverage; final art deferred)
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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