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"]

runtime/src/lib.rs

@@ -6,7 +6,6 @@ extern crate alloc;
 #[cfg(feature = "std")]
 extern crate std;
 
-use bytes::BufMut;
 use core::fmt;
 
 pub struct MapFieldIterator<'a> {
@@ -64,6 +63,7 @@ impl std::error::Error for RotoError {}
 
 pub type Result<T> = core::result::Result<T, RotoError>;
 
+#[cfg(feature = "alloc")]
 pub trait RotoOwned {
     type Reader<'a>
     where
@@ -71,6 +71,7 @@ pub trait RotoOwned {
     fn reader(&self) -> Self::Reader<'_>;
 }
 
+#[cfg(feature = "alloc")]
 pub trait RotoMessage: Sized {
     fn decode(buf: bytes::Bytes) -> Result<Self>;
     fn bytes(&self) -> bytes::Bytes;
@@ -1035,10 +1036,15 @@ impl<'a> ProtoBuilder<'a> {
     }
 }
 
+#[cfg(feature = "alloc")]
+use bytes::BufMut;
+
+#[cfg(feature = "alloc")]
 pub struct BufMutBuilder<'a, B: BufMut> {
     buf: &'a mut B,
 }
 
+#[cfg(feature = "alloc")]
 impl<'a, B: BufMut> BufMutBuilder<'a, B> {
     pub fn new(buf: &'a mut B) -> Self {
         Self { buf }