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/no_std_test/Cargo.toml

@@ -3,7 +3,13 @@ name = "no_std_test"
 version = "0.1.0"
 edition = "2021"
 
+[lib]
+crate-type = ["staticlib"]
+
 [dependencies]
 roto-runtime = { path = "../../runtime", default-features = false }
-prost = "0.13"
-bytes = "1.8"
+bytes = { version = "1.8", default-features = false, optional = true }
+
+[features]
+default = ["alloc"]
+alloc = ["roto-runtime/alloc", "bytes"]