Per user direction: feature-gating is appropriate for per-step diagnostic
logging (real perf/memory cost) but the name should be specific to the
mechanism, not a generic "diag-log" catch-all. kernel-step-trace
describes what the feature provides — per-step records written to the
ring by kernels.
Pure rename, no behavioral changes. Touched: Cargo.toml feature decl,
lib.rs cfg attribute, perception.rs (~25 cfg attributes including
not(feature) pairs), gpu_log.rs + smoothness_lambda_controller.cu doc
comments, gpu_log_ring_invariants.rs file-level cfg + ignore-attribute
text.
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