review: iteration-2 fixes (fragment assertion, evidence paths, hardening)

- build-kernel.sh fragment assertion: survives a missing trailing newline
  (the read footgun, reproduced) and treats an absent symbol on a disable
  line as FATAL, symmetric with the enable arm.
- fetch-kernel-tarball.sh checks the pin before downloading; both fetchers
  add --retry-connrefused.
- mkimage rejects '.'/'..' state keys.
- ui-shot: VM liveness checked before every QMP call, console.log preserved
  as evidence on every failure path, repaint deadline widened to 90s with
  the contended-runner rationale documented.
- rs485-bridge: overflow discards back off one gap and rate-limit their log
  line, mirroring the accept-loop fix; clippy nit fixed.
- .gitignore ignores *.elf/*.map so the untracked artifacts cannot silently
  return; CI shellcheck glob now covers build/ and the rootfs boot scripts
  (directives added for the deliberate in-guest source paths).
- Docs: NPU parity row matches its sibling verification docs; line-pinned
  audit cross-references unpinned; CROSS_COMPILE documented in the build
  header; payload README lists warden-ui; ci-cd tense settled.

Verified: guards negative-tested (bad state keys, no-newline fragment);
boot smoke, portal scenario, ui-shot all PASS; 53 tests green; shellcheck
clean across the widened glob; clippy zero.

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

qemu/mkimage.sh

@@ -36,6 +36,9 @@ while [ $# -gt 0 ]; do
         *) echo "FATAL: --state needs KEY=VALUE, got '$2'" >&2; exit 1 ;;
       esac
       case "${2%%=*}" in
+        .|..)
+          echo "FATAL: --state key cannot be '.' or '..'" >&2
+          exit 1 ;;
         *[!A-Za-z0-9_.]*|'')
           echo "FATAL: --state key '${2%%=*}' must match [A-Za-z0-9_.]+ (it becomes a filename)" >&2
           exit 1 ;;