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/rs485-bridge/src/lib.rs

@@ -61,6 +61,7 @@ pub fn pump_serial(
     stream.set_read_timeout(Some(gap))?;
     let mut buf: Vec<u8> = Vec::new();
     let mut chunk = [0u8; 256];
+    let mut discards: u64 = 0;
     loop {
         match (&*stream).read(&mut chunk) {
             Ok(0) => {
@@ -72,12 +73,20 @@ pub fn pump_serial(
             Ok(n) => {
                 buf.extend_from_slice(&chunk[..n]);
                 if buf.len() > MAX_PENDING {
-                    eprintln!(
-                        "rs485: {} bytes buffered with no inter-frame gap — discarding \
-                         (misbehaving master streaming continuously?)",
-                        buf.len()
-                    );
+                    // Rate-limit the log and back off for one gap so a master
+                    // streaming continuously cannot peg a core and flood
+                    // stderr — mirroring the accept-loop backoff.
+                    discards += 1;
+                    if discards == 1 || discards.is_multiple_of(256) {
+                        eprintln!(
+                            "rs485: {} bytes buffered with no inter-frame gap — \
+                             discarding (misbehaving master? {} discards so far)",
+                            buf.len(),
+                            discards
+                        );
+                    }
                     buf.clear();
+                    std::thread::sleep(gap);
                 }
             }
             Err(e)