feat(rail-drop): Plinko-on-a-rail casino game

[naosbtc]

Summary

A new Stake Engine game where a ball rolls along a horizontal rail and drops into one of three Plinko boards positioned along its path. Each zone has progressively more rows (8 / 12 / 16) and more extreme multipliers, modeled on Stake Plinko's published distributions.

Multiplier tables (~99% RTP per zone)

Zone	Rows	Low	Medium	High
1 (near)	8	5.6× — 0.5×	13× — 0.4×	29× — 0.2×
2 (mid)	12	10× — 0.5×	33× — 0.3×	170× — 0.2×
3 (far)	16	16× — 0.5×	110× — 0.3×	1000× — 0.2×

Each zone uses a binomial slot distribution; multipliers match the values from Stake's published Plinko paytables.

What's included

• Full Stake SDK app scaffold under `apps/rail-drop/` (workspace deps: `config-vite`, `config-svelte`, `config-ts`, `config-lingui`).
• Book events: `drop` (zone, riskLevel, path, slotIndex, multiplier) and `finalWin`.
• Animated SVG scene (1600×900): rail with arrow, drop indicators per zone, plinko boards with pins + colored slots, ball physics (rolling → dropping → row-by-row bouncing → landing), win overlay with multiplier-based color coding.
• 3 risk-level selector (low/medium/high), in-canvas SVG buttons.
• Six sample books under `stories/data/sampleBooks.ts` covering each zone × risk combo (loss → max win).
• Dev panel inside `Game.svelte` to play sample books without an RGS session.

Test plan

• `pnpm install` resolves `apps/rail-drop`'s workspace deps
• `pnpm --filter=rail-drop build` succeeds (verified locally — `✓ built in 2.83s`, prerender `200 /`)
• `pnpm --filter=rail-drop dev` boots on port 3002
• Each sample book in the dev panel plays the full animation (rolling → bounce → land → win overlay)
• Risk level buttons change displayed slot multipliers when phase is idle
• Math-sdk Python implementation (separate PR) emits matching `drop` events

Notes

• This PR contains the frontend only. A matching math-sdk Python game (`games/0_0_rail_drop/`) needs to be authored to generate `BookEventDrop` events server-side.
• Per-zone selection probability is server-controlled — the frontend just renders whatever zone the server picks.

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