qemu: RS485 bridge to sim, portal E2E scenario, CI wiring

Phase 3 of the device sim, all verified on QEMU 10.0.11:

- qemu/rs485-bridge/: std-only crate bridging a QEMU serial chardev (unix
  socket) to warden_sim::ModbusSlave — gap-based RTU framing (CRC failures
  degrade to real-slave silence), line-protocol control socket for register
  seeding and fault injection (drop/exception/clear), bounds-checked so a
  scenario typo answers err instead of panicking the bus. 7 unit tests,
  bench in the sim_bench pattern. Verified end-to-end: guest master frame
  on /dev/ttyS4 (pci-serial) answered from the sim slave, CRC-correct.
- virt machine gains highmem=off: the 32-bit non-LPAE kernel cannot reach
  virt's default 40-bit PCIe ECAM (pci-host-generic EOVERFLOW); with it the
  full PCI set probes (16550A ttyS0, i6300esb).
- Watchdog scenario verified: guest arms /dev/watchdog, no petting, i6300esb
  resets the VM ~30s later (first environment where this arm is testable).
- qemu/tests/portal-scenario.sh: the real static-musl warden-flared inside
  the VM against flare-edge's mock portal on the host — authenticated
  check-in, firmware desired-state pull, and download of a real signed
  tier-1 .wfw offer, asserted from the portal log. Found and filed
  flare-edge#106 (fatal SIGBUS in the HPMCU boot-loaded probe on
  non-RV1106 memory maps); runs against a flared built from the
  qemu-vm-support fix branch.
- stage-2 init: WARDEN_FLARE_INSECURE=1 + WARDEN_HPMCU=0 (documented VM
  deviations), firmware-version stamp, newline-terminated state seeds.
- CI: rs485-bridge joins the test loop and bench job; new qemu-tools job
  (shellcheck + initramfs + disk image on hosted runners); kernel-build
  gains a fail-closed qemu boot-smoke step. All qemu scripts shellcheck-clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018HUayid7W5w7jBdb9Rrj1K

qemu/rs485-bridge/Cargo.toml

@@ -0,0 +1,26 @@
+[package]
+name = "warden-rs485-bridge"
+version = "0.1.0"
+edition = "2021"
+description = "Bridges a QEMU serial chardev (unix socket) to the warden-sim Modbus RTU slave, so the VM guest's RS-485 master polls the same simulated field bus the unit tests do — fault injection included."
+license = "MIT OR Apache-2.0"
+
+[lib]
+name = "warden_rs485_bridge"
+path = "src/lib.rs"
+
+[[bin]]
+name = "rs485-bridge"
+path = "src/main.rs"
+
+# Std-only on purpose: the sim crate it wraps is zero-dep, and unix sockets +
+# read timeouts need nothing external.
+[dependencies]
+warden-sim = { path = "../../sim" }
+
+[dev-dependencies]
+
+# Dependency-free micro-benchmarks, same pattern as sim/benches/sim_bench.rs.
+[[bench]]
+name = "bridge_bench"
+harness = false