test(perf): measure the serial IPC depth before the J1 first card

Adds renderer.ipcSerialDepthToFirstCard to the launch journey: the length
of the longest chain of bridge calls in which each call started after the
previous one completed, among calls that completed before the first card.
The main IPC capture now records the ordered start/completion timeline;
the depth, its lower bound, the chain and the timeline are per-iteration
evidence, and the CI job summary prints the chain.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/actions/performance-journeys/action.yml

@@ -70,5 +70,7 @@ runs:
                           ($j.counterStability[.key] // {}) as $s |
                           "| `\(.key)` | \(.value) | \(($s.values // []) | map(tostring) | join(", "))\(if $s.stable == false then " (unstable)" else "" end) |"),
                       (($j.wallClock // {}) | to_entries[] | "| `\(.key)` | \(.value) | |"),
-                      "")
+                      "",
+                      ([($j.iterations // [])[] | select(.warmup | not) | .evidence.ipcSerialDepth // empty][0] // empty |
+                          "Serial IPC chain (first measured iteration): \(.chain | map("`\(.)`") | join(" → "))", ""))
               ' "$SUMMARY" | tee -a "$GITHUB_STEP_SUMMARY"