Move kaintuck-server crate from server/ into crates/

#53 in Riparion/riparion-retro — merged 2026-06-15

The package was already named kaintuck-server; only its directory was server/, listed explicitly in the workspace members. This moves it under crates/ so it lives alongside the other crates (picked up by the crates/* glob) and drops the now-redundant "server" entry.

All justfile recipes (just server, just play, just smoke) and scripts/smoke-server.sh reference the package name via -p kaintuck-server, not a path, so nothing else needed updating. cargo check -p kaintuck-server passes.

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