feat: gate alloc/std features for no_std/no_alloc support

Gate generated code behind feature flags so consumers can build in
no_std/no_alloc environments:

- Runtime: gate RotoMessage, RotoOwned, BufMutBuilder behind alloc
- Codegen: split DATA_IMPORTS, gate Owned* structs + service code
- Fix RevBuilder::with() to iterate forward without Vec allocation
- Wire up bytes/std via alloc feature (Bytes requires Arc)
- Update consumer Cargo.toml files with proper feature chains
- Add no_std_test example for embedded validation

examples/hello_world/Cargo.toml

@@ -31,5 +31,6 @@ tonic-build = "0.12"
 roto-codegen = { path = "../../codegen" }
 
 [features]
-default = ["alloc"]
-alloc = []
+default = ["std"]
+std = ["roto-runtime/std", "alloc"]
+alloc = ["roto-runtime/alloc"]