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

@@ -17,9 +17,10 @@ serde      = { version = "1", optional = true }
 serde_json = { version = "1", optional = true }
 
 [features]
-smarm-trace = ["smarm/smarm-trace"]
-channels    = []
-phoenix     = ["channels", "dep:serde", "dep:serde_json"]
+smarm-trace  = ["smarm/smarm-trace"]
+smarm-causal = ["smarm/smarm-causal"]
+channels     = []
+phoenix      = ["channels", "dep:serde", "dep:serde_json"]
 
 [dev-dependencies]
 serde      = { version = "1", features = ["derive"] }