feat: smarm-causal passthrough feature

Mirrors the smarm-trace precedent: urus itself gains no causal code
paths yet — this just lets downstream binaries (the Phase 2 causal
bench lives in examples/) flip smarm's instrumentation on through the
urus dep. Whole suite is green with the feature enabled: the causal
runtime is behaviorally inert while no experiment is active.

Cargo.toml

@@ -18,6 +18,7 @@ serde_json = { version = "1", optional = true }
 
 [features]
 smarm-trace  = ["smarm/smarm-trace"]
+smarm-causal = ["smarm/smarm-causal"]
 channels     = []
 phoenix      = ["channels", "dep:serde", "dep:serde_json"]