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

Cargo.lock

@@ -1159,6 +1159,7 @@ checksum = "dc897dd8d9e8bd1ed8cdad82b5966c3e0ecae09fb1907d58efaa013543185d0a"
 name = "roto-benches"
 version = "0.1.0"
 dependencies = [
+ "bytes",
  "clap",
  "criterion",
  "env_logger",