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

benches/Cargo.toml

@@ -4,10 +4,13 @@ version = "0.1.0"
 edition = "2024"
 
 [features]
-alloc = []
+default = ["std"]
+std = ["roto-runtime/std", "alloc"]
+alloc = ["roto-runtime/alloc"]
 
 [dependencies]
 roto-runtime = { path = "../runtime" }
+bytes = "1.7"
 clap = { version = "4", features = ["derive"] }
 log = "0.4"
 env_logger = "0.11"