build(examples): gate load_profile behind smarm-causal

smarm::causal is feature-gated, so examples/load_profile.rs no longer
compiles under the default feature set and broke `cargo build --examples`.
Declare it with required-features = ["smarm-causal"] so the default build
skips it and `--features smarm-causal` still builds it.

Splits the committable half out of the working-tree Cargo.toml; the
`smarm = { path = "../smarm_full" }` dev pin stays uncommitted and is
restored to a tag in the release commit.

Cargo.toml

@@ -54,3 +54,7 @@ required-features = ["phoenix"]
 [[example]]
 name = "serve_toml"
 required-features = ["config-file"]
+
+[[example]]
+name = "load_profile"
+required-features = ["smarm-causal"]