Add a global style guide

STYLES.md

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+# Style Guide for all of Luminescent Dreams projects
+
+**Many projects predate this style guide. Future work should follow this style guide. Previous work should be updated on an opportunistic basis.**
+
+## Prefer let/else bindings over match
+
+If the point of a match statement is to unwrap a value or exit early, prefer let/else bindings to a match statement:
+
+Avoid:
+```rust
+let game = match db.game(&image.game)? {
+    Some(game) => game,
+    None => return Ok(Err(Error::NotFound(image.game.as_str().to_string()))),
+};
+```
+
+Prefer:
+```rust
+let Some(game) = db.game(&image.game)? else {
+    return Ok(Err(Error::NotFound(image.game.as_str().to_string())));
+};
+```
+
+## Use nested results to separate safe failures from critical failures
+
+[Error Handling in a Correctness-Critical Rust Project | sled-rs.github.io](https://sled.rs/errors.html)
+
+If a function could return a combination of ordinary errors and fatal errors, separate those into separate error types and nest them.
+
+Avoid:
+```rust
+fn fallible() -> Result<(), BlobError>
+```
+
+Prefer:
+```rust
+fn fallible() -> Result<Result<(), Error>, Fatal>
+
+fn search() -> Result<Option<()>, Fatal>
+
+fn fatal_only() -> Result<(), Fatal>
+```
+
+Ordinary errors should never be blanket-promoted to fatal errors, and fatal errors should never be caught except at the top level of the application. Functions receiving a fatal error MAY choose to perform cleanup before propogating the error further.