Add project rule: prefer msgspec.Struct over dataclasses (#28816)

.claude/rules/no-dataclasses.md

@@ -0,0 +1,24 @@
+---
+paths:
+  - "**/*.py"
+---
+
+# Use `msgspec.Struct`, not `@dataclass`
+
+Define data containers as `msgspec.Struct`. Do not add new
+`dataclasses.dataclass` (or `attrs`) — they weaken strict type checking and
+don't map onto Rust structs for the planned Rust migration.
+
+```python
+import msgspec
+
+class LoadSnapshot(msgspec.Struct):   # frozen=, kw_only=, omit_defaults= as needed
+    dp_rank: int = 0
+    tokens: list[int] = []            # mutable defaults are safe
+```
+
+- Methods / `@classmethod` constructors go on the `Struct`; see
+  `python/sglang/srt/managers/load_snapshot.py`.
+- New code only. Existing `@dataclass` is grandfathered — migrate opportunistically
+  while editing the file, not in drive-by sweeps.
+- If a third-party API forces `@dataclass`, keep it at that boundary only.