add arbitrary selection mechanic to repo

arbitrary_selection_mechanics.md

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+# Arbitrary Selection Mechanics
+
+## Coin
+
+- Heads: `0`
+- Tails: `1`
+
+## Dice Mapping
+
+| Die Value | Coin Flip Encoding |
+|-----------|--------------------|
+| `1`       | `100`              |
+| `2`       | `010`              |
+| `3`       | `110`              |
+| `4`       | `001`              |
+| `5`       | `101`              |
+| `6`       | `011`              |
+| `7`       | `111`              |
+| `8`       | `000`              |
+
+## Background
+
+- coin flip values based on binary numbers
+- `0` is highest value on selection lists
+- encodings are indexed starting at `1`
+- coin forces `8` possible values
+  - `111` and `000` are "special options"
+  - `free choice` as option
+  - `re-roll` as option
+
+## Mechanic
+
+- list and number options
+  - up to `6` or `8` based on die/coin selection
+  - if half as many, give each choice multiple numbers
+- roll die once or flip coin 3 times
+  - note/remember each coin flip result
+- lookup choice based on value obtained
+- act upon the choice
+
+## Example Lists
+
+### Yes / No / Maybe
+
+| Value    | Choice |
+|----------|--------|
+| `1`, `4` | Yes    |
+| `2`, `5` | No     |
+| `3`, `6` | Maybe  |
+
+## Activity
+
+| Value    | Choice        |
+|----------|---------------|
+| `1`, `5` | Park          |
+| `2`, `6` | Movie         |
+| `3`, `7` | Coffee        |
+| `4`, `8` | Nature Center |
+
+## Adult Time
+
+| Value              | Choice          |
+|--------------------|-----------------|
+| `1`, `3`, `5`, `7` | Dinner / Movie  |
+| `2`, `4`, `6`, `8` | Netflix / Chill |