kaintuck: shared-economy server + gossip, Faro & Vingt-et-Un minigames

#51 in Riparion/riparion-retro — merged 2026-06-14

Brings the kaintuck-gossip-river-only line of work to main. This branch is 10 commits ahead; the highlights:

Shared-economy multiplayer (kaintuck)

• retro-core: extracted pure rng/format/hash + trail-kit gossip schema
• kaintuck-engine: headless engine + shared-economy sim + trader gossip
• kaintuck-server: native axum/WebSocket shared-economy server (+ smoke recipes)
• kaintuck UI: opt-in multiplayer join client; actionable market banter + dockside rumor mechanic; Trace hazard retuning

Minigames-kit card games

• Faro: seeded banking card game + standalone demo (examples/faro)
• Vingt-et-Un (this session): twenty-one against a fixed dealer (stands on all 17s, naturals pay 3:2), pure unit-tested helpers, standalone demo (examples/vingt-un)
• Shared cards module extracted from the two card games (rank/deck constants, card-face styling, Fisher–Yates shuffle, stake-progression end check, FlipCard), mirroring the existing grid module

Verification

• cargo test -p minigames-kit → 75 pass (incl. 13 new Vingt-et-Un cases)
• Whole-workspace cargo check; dx build of the vingt-un demo (wasm + assets) succeeds

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