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

runtime/Cargo.toml

@@ -4,9 +4,9 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
-bytes = { version = "1.7", default-features = false }
+bytes = { version = "1.7", default-features = false, optional = true }
 
 [features]
-default = ["std", "alloc"]
-std = []
-alloc = []
+default = ["std"]
+std = ["alloc"]
+alloc = ["bytes/std"]