Add alchemy tower README

docs/gyms/tiny_sword/buildings/alchemy_tower/README.md

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+# Alchemy Tower
+
+The Alchemy Tower is a building that passively produces **magic gold** and allows **crafting** other currencies through alchemical recipes.
+
+## Passive Production
+
+The tower runs a continuous production cycle. Once a cycle completes, magic gold is added to your balance and the next cycle begins with a slightly longer duration.
+
+| Data field | Default | Description |
+|---|---|---|
+| `base_production_time_seconds` | 5.0 | Duration of the first cycle (seconds) |
+| `production_time_growth_multiplier` | 1.2 | Each completed cycle multiplies the next cycle's duration by this amount |
+| `magic_gold_per_cycle` | 1.0 | Base magic gold produced per cycle |
+
+Production is only active when at least one worker is assigned and the tower is available in the game state.
+
+## Workers
+
+Workers are assigned by spending 1 **worker** currency per worker. Each assigned worker provides a **+10% speed bonus** to production (configurable via `base_worker_speed_bonus`).
+
+- Max workers: `max_workers` (default 10)
+- The speed multiplier formula: `1.0 + (worker_count * base_worker_speed_bonus)`
+- This multiplier affects both the rate time advances and the final cycle output
+
+The UI panel includes a worker counter and an "Assign Worker" button. The button is disabled when you have no workers available or have reached the max.
+
+## Crafting
+
+The crafting panel shows recipes from an `AlchemyCraftCatalogue` resource. Each recipe (`AlchemyCraftRecipe`) defines:
+
+- `id` — unique recipe identifier
+- `output_currency` / `output_amount` — what you get
+- `cost_entries` — array of `CurrencyCostEntry` resources, each specifying a currency and an amount
+
+To craft, click the recipe's button. The tower deducts all costs from your balance via `game_state.spend_currency()`. If any cost can't be covered, the craft fails silently and nothing is deducted. On success, the output currency is added to your balance.
+
+## Signals
+
+| Signal | Emitted when |
+|---|---|
+| `production_progress_updated(progress: float)` | Every frame during production; 0.0–1.0 |
+| `production_completed(amount: BigNumber)` | A production cycle finishes |
+| `magic_gold_balance_updated(amount: BigNumber)` | Magic gold balance changes |
+
+## Key Files
+
+| File | Purpose |
+|---|---|
+| `alchemy_tower.gd` | Main tower logic: production loop, worker management, crafting |
+| `alchemy_tower_data.gd` | Configuration resource (`AlchemyTowerData`) |
+| `alchemy_craftable_panel.gd` | UI panel (`AlchemyCurrenciesPanel`) for progress bar, balance, workers, and recipe list |
+| `alchemy_craftable_panel_tile.gd` | Individual recipe tile in the crafting list |
+| `recipes/alchemy_craft_catalogue.gd` | Catalogue resource holding all available recipes |
+| `recipes/alchemy_craft_recipe.gd` | Single recipe resource |
+| `recipes/currency_cost_entry.gd` | Cost entry for a single currency in a recipe |