bench: dependency-free sim micro-benchmarks + CI bench job (P4)

benches/sim_bench.rs (harness=false, zero deps): fixed-iteration ns/op timing for
hpmcu_tick, cru_poll, modbus_read_holding, rga_improcess, membus_poke_peek. Human
timings to stdout, one JSON trend line per bench to stderr for CI capture. New CI
`bench` job smoke-runs them. Regression-vs-history gating is future work.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017wB8KB3MMQztRDXCMCkPrf

sim/Cargo.toml

@@ -12,3 +12,8 @@ path = "src/lib.rs"
 [dependencies]
 
 [dev-dependencies]
+
+# Dependency-free micro-benchmarks (fixed-iteration timing, no criterion tree).
+[[bench]]
+name = "sim_bench"
+harness = false