perf(csv-loader): optimize reverse reference resolution

Implement a reverse lookup cache to avoid re-filtering the entire
referenced table for every row during reverse reference resolution.
This improves performance from O(N*M) to O(N+M) where N is the number
of rows in the current table and M is the number of rows in the
referenced table.

Also update documentation to reflect the new union resolution
behavior and migration notes for version 2.0.0.

README.md

@@ -139,6 +139,8 @@ The following TypeScript types are exported:
 | Int | `int` | `42` |
 | Float | `float` | `3.14` |
 | Number | `number` | `42` or `3.14` |
+
+> **Note:** `int`, `float`, and `number` all collapse to the `number` type in generated TypeScript declarations (`schemaToTypeString`). The distinction is preserved at parse time — `int` rejects non-integer values while `float`/`number` accept them.
 | Boolean | `boolean` | `true` or `false` |
 | Tuple | `[Type1; Type2; ...]` | `[hello; 42; true]` |
 | Array | `Type[]` | `[1; 2; 3]` |
@@ -159,6 +161,16 @@ The following TypeScript types are exported:
 - In CSV schema rows, double-quoted string literals like `"active" | "inactive"` are handled automatically by the loader; avoid commas inside string literals (use single-quoted literals like `'a,b'` for those)
 - For CSV loading with reference resolution, see [csv-loader.md](./csv-loader.md)
 
+## Migration from 1.x
+
+Version 2.0.0 introduces breaking changes to the schema DSL:
+
+1. **Composite values must be fully bracketed.** Tuple and array values now always require `[]` — `[a; 1]; [b; 2]` is no longer accepted; write `[[a; 1]; [b; 2]]`.
+2. **The `[Type][]` array form is removed.** Use `Type[]` (e.g. `[string; number][]` instead of `[[string; number]][]`).
+3. **`[single]` is now a 1-tuple, not an array.** Use `Type[]` for single-element arrays.
+4. **Union resolution prefers reference members.** In a union containing references, reference members are tried before non-reference members regardless of author order (see the notes above).
+5. **Schema cells may be fully quoted.** Double-quoted string literals like `"active" | "inactive"` in a CSV schema row are handled automatically by the loader.
+
 ```javascript
 // rspack.config.js
 module.exports = {