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.lock

@@ -0,0 +1,24 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "bytes"
+version = "1.12.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8ae3f5d315924270530207e2a68396c3cc547f6dca3fbdca317cfb1a51edb593"
+
+[[package]]
+name = "no_std_test"
+version = "0.1.0"
+dependencies = [
+ "bytes",
+ "roto-runtime",
+]
+
+[[package]]
+name = "roto-runtime"
+version = "0.1.0"
+dependencies = [
+ "bytes",
+]