chore(docs): document synthetics directory structure (#1393)

I'm not entirely sure this is correct, but I thought this would be more
useful to the project than just asking in slack.

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: michaelkirk

test/synthetic/README.md

@@ -0,0 +1,30 @@
+# Synthetic MLT Test Fixtures
+
+To verify encoder and decoder implementations across languages, this directory
+contains interesting `.mlt` tiles along with their expected logical
+representation as JSON.
+
+## Encoding
+
+### `0x01/` — Java reference
+
+Generated by `SyntheticMltGenerator.java`. These are the canonical files; the
+Rust encoder verifies byte-for-byte against them where possible.
+
+### `0x01-rust/` — Rust-only output
+
+Generated by the `mlt-synthetics` crate. Files land here when the Rust encoder
+produces valid output that differs from Java's bytes. MLT is complex enough
+that there are often multiple reasonable ways to encode.
+
+## Decoding
+
+Any correct decoder should produce identical logical output (json) from the
+same `.mlt`, regardless of which encoder wrote it. So Java and TypeScript
+decoding a file from `0x01-rust/` should produce the same `.json` as Rust did.
+
+## Where to add a new synthetic
+
+- **`0x01/`** — Java can encode it. Add to `SyntheticMltGenerator.java`.
+- **`0x01-rust/`** — Java can't or won't produce it. Add to `mlt-synthetics/src/main.rs` with a
+  `-rust` suffix in the name (the tool strips it when writing the file).