diag(quic-interop): boot log + build-id verify deployed image is fresh

The [batch]/[interop] traces aren't appearing in the user's runs
even after rebuild. To distinguish 'env var not set' from 'binary
doesn't have the code', emit a [boot] line at startup that:

  1. Reports the QUIC_INTEROP_DEBUG env var value
  2. Reports whether writerDebugEnabled was flipped on
  3. Includes a build-id constant from WriterDebug.kt

If the user's run shows '[boot] DEBUG=1; ... build_id=2026-05-07-
batch-log-v1', we know the latest debug code IS deployed. If the
boot line is missing OR shows an older build_id, the docker image
served stale bytecode (gradle/docker layer caching) and a clean
rebuild is needed.

Inspect script now greps [boot] and surfaces it BEFORE the rest of
the writer-side debug section.

quic/interop/inspect-multiplexing.sh

@@ -38,7 +38,12 @@ echo "=============== writer-side debug traces (DEBUG=1 build only) ============
 # `grep -c` exits 1 on no matches AND prints "0", so a naive
 # `grep -c ... || echo 0` doubles up. Suppress the exit code
 # instead.
-WRITER_LINES=$(grep -cE '\[(writer|batch|interop)' "$CASE_DIR/output.txt" 2>/dev/null) || WRITER_LINES=0
+echo
+echo "=============== boot line (verifies image has latest debug build) ==============="
+grep '\[boot\]' "$CASE_DIR/output.txt" 2>/dev/null | head -n 5 \
+    || echo "(no [boot] lines — image is older than the boot log; rebuild with: DEBUG=1 ./quic/interop/run-matrix.sh ...)"
+
+WRITER_LINES=$(grep -cE '\[(writer|batch|interop|boot)' "$CASE_DIR/output.txt" 2>/dev/null) || WRITER_LINES=0
 if [[ "$WRITER_LINES" -gt 0 ]]; then
     echo "($WRITER_LINES diagnostic lines; [batch] / [interop] entries first then first 30 [writer.app]:)"
     # Use `|| true` because grep returns 1 on no matches and the

quic/interop/src/main/kotlin/com/vitorpamplona/quic/interop/runner/InteropClient.kt

@@ -85,8 +85,15 @@ private const val MULTIPLEX_PARALLELISM = 64
 fun main() {
     // Single env-var check, propagated to library code that opts into
     // verbose tracing only when this is set.
-    if (System.getenv("QUIC_INTEROP_DEBUG") == "1") {
+    val debugEnv = System.getenv("QUIC_INTEROP_DEBUG")
+    if (debugEnv == "1") {
         com.vitorpamplona.quic.connection.writerDebugEnabled = true
+        System.err.println(
+            "[boot] DEBUG=1; writerDebugEnabled=true; build_id=" +
+                "${com.vitorpamplona.quic.connection.WRITER_DEBUG_BUILD_ID}",
+        )
+    } else {
+        System.err.println("[boot] DEBUG=${debugEnv ?: "(unset)"} writerDebugEnabled=false")
     }
 
     val role = System.getenv("ROLE") ?: "client"