feat: bootstrap walking skeleton

docs/architecture.md

@@ -0,0 +1,58 @@
+# Architecture
+
+Idlegame is split into four layers.
+
+## Engine
+
+`src/engine/` contains deterministic gameplay logic:
+
+- `tickLoop.ts`: fixed-timestep accumulator. Large elapsed deltas, including
+  offline catch-up, drain through the same tick path as active play.
+- `game.ts`: core game state, active action progress, and resource accrual.
+- `save.ts`: versioned save schema, serialized export/import strings, and
+  offline elapsed calculation.
+- `num.ts`: branded numeric boundary and human-readable formatting.
+
+The engine must stay pure. It does not import React, Zustand, browser APIs,
+storage, requestAnimationFrame, or Date scheduling.
+
+## Content
+
+`src/content/` contains authored definitions validated by Zod. M0 includes one
+resource and one timed action. M1 expands this into real resources, actions,
+story nodes, automation unlocks, and prestige definitions.
+
+## State
+
+`src/state/` owns environment coupling:
+
+- persistence backend selection: IndexedDB via `idb-keyval`, with localStorage
+  fallback
+- load-on-boot and autosave orchestration
+- requestAnimationFrame loop and lifecycle hooks
+- mapping engine state to view models
+- Zustand store updates for React
+
+## UI
+
+`src/ui/` renders the view model and calls runtime commands. Components should
+not implement gameplay rules. The M0 shell includes a resource readout, action
+panel, progress bar, and event log.
+
+## Verification
+
+Run the full local gate before pushing:
+
+```powershell
+pnpm typecheck
+pnpm lint
+pnpm test:coverage
+pnpm build
+```
+
+Rendered checks for the walking skeleton:
+
+- app loads without framework overlay or console errors
+- starting the timed action increments Gold after completion
+- reload preserves saved state
+- reopening after time away credits offline progress