review: iteration-1 fixes across CI, bridge, VM harness, and docs

CI/pipeline:
- KERNEL_TARBALL passed as a YAML env literal '~' was never tilde-expanded
  and would have failed every hosted kernel-build dispatch; the path is now
  exported from the shell. Verified reproducible before the fix.
- Every job gets timeout-minutes; boot smoke uses timeout -k so a wedged
  qemu is SIGKILLed instead of holding the job.
- Tarball fetch + fail-closed sha256 verification deduplicated into
  build/fetch-kernel-tarball.sh (with curl retries), used by build-kernel.sh
  and both CI jobs. busybox fetch gains retries too.
- ccache layer for kernel-build (cache keyed on defconfig+patches) recovers
  the incremental-compile speed the ephemeral-runner move cost.
- build-kernel.sh now asserts every fragment option survived olddefconfig —
  merge_config -m pastes text and Kconfig silently drops unmet symbols.

rs485-bridge:
- pending-buffer cap (2x max RTU ADU) instead of unbounded growth;
  explicit accept-loop error handling with backoff instead of .flatten();
  per-arm inline bounds instead of the string-keyed lookup whose default
  would have mis-bounded a future get-input; control-socket cleanup errors
  surfaced; flag-shaped values rejected in arg parsing; doc example uses a
  private mktemp dir. Test timing margins widened for contended runners
  (gap 25->120ms, 60x margin on the split-frame test).

VM harness:
- stage-1/stage-2 boot scripts share one validated slot parser and one
  by-name populator (qemu/rootfs/etc/warden-lib.sh) — the duplicated
  parser had already diverged on validation; userdata/oem mount failures
  now fail fast with a greppable sentinel; udhcpc fallback keys off the
  interface actually having an address; switch_root applet guarded.
- boot-smoke delegates the qemu invocation to run.sh (machine shape lives
  in ONE place); run.sh port 0 disables a hostfwd.
- mkimage: unknown partition names fail at build time; DISK_END is a max,
  not last-entry; --state keys validated as filenames.
- portal-scenario: mock readiness is asserted (no silent fall-through),
  hostfwd port collisions retried, mount-failure sentinel fails fast.
- ui-shot: fixed sleeps replaced with bounded screendump polling; the
  repaint assertion is real and documented as such. qmp.py loses its
  module-global and gains argv validation.

Docs/scrub: bench-host paths and the site AP name removed from six more
port docs and two evidence tables; path-bearing build artifacts (.elf,
.map) untracked (the 154-byte firmware .bin is path-free and stays);
ADR-0003 marked visibility-superseded by ADR-0007; stale section
cross-reference fixed; flare-edge noted as private for outside readers;
stale root-level review report removed per the new workspace rule.

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

qemu/rootfs/etc/rc

@@ -1,37 +1,19 @@
 #!/bin/busybox sh
 # Stage-1 rc: sourced by /init (still PID 1, initramfs root) when a virtio
-# disk is present. Emulates U-Boot's slot choice — parse warden.slot= from the
-# cmdline, mount that rootfs, switch_root into it. This is an EMULATION of the
-# A/B selection outcome, not the BCB/bootcount mechanism itself.
+# disk is present. Emulates U-Boot's slot choice — mount the validated slot's
+# rootfs and switch_root into it. This is an EMULATION of the A/B selection
+# outcome, not the BCB/bootcount mechanism itself.
+#
+# Every guarded failure path `return`s to /init (valid in a sourced script;
+# /init then falls through to shell/poweroff). The final exec is the one
+# unguardable step: if switch_root itself fails to launch, the shell — PID 1 —
+# exits and the kernel panics; the applet-existence check below catches the
+# only preventable variant of that.
 
-# /dev/block/by-name/<PARTNAME> symlinks: the contract flare-edge slotctl.rs
-# relies on. blkdevparts= gives every vda partition a PARTNAME in sysfs.
-mkdir -p /dev/block/by-name
-for uev in /sys/class/block/vda*/uevent; do
-    [ -f "$uev" ] || continue
-    partname=""
-    devname=""
-    while IFS='=' read -r k v; do
-        case "$k" in
-            PARTNAME) partname="$v" ;;
-            DEVNAME)  devname="$v" ;;
-        esac
-    done < "$uev"
-    [ -n "$partname" ] && [ -n "$devname" ] \
-        && ln -sf "/dev/$devname" "/dev/block/by-name/$partname"
-done
+. /etc/warden-lib.sh
 
-# Slot select: whole-token parse of warden.slot= (never a substring match).
-slot="_a"
-for tok in $(cat /proc/cmdline); do
-    case "$tok" in
-        warden.slot=*) slot="${tok#warden.slot=}" ;;
-    esac
-done
-case "$slot" in
-    _a|_b) ;;
-    *) echo "rc: bad warden.slot='$slot', falling back to _a"; slot="_a" ;;
-esac
+warden_populate_by_name
+slot="$(warden_slot)"
 
 root="/dev/block/by-name/rootfs${slot}"
 if [ ! -e "$root" ]; then
@@ -49,6 +31,11 @@ if [ ! -x /mnt/sbin/init ]; then
     umount /mnt
     return 0
 fi
+if ! command -v switch_root >/dev/null; then
+    echo "rc: busybox lacks switch_root — staying in initramfs"
+    umount /mnt
+    return 0
+fi
 
 echo "rc: switching root to rootfs${slot} ($root)"
 exec switch_root /mnt /sbin/init