test(performance): add J4 search journey

Measures typing a six-character query into the header search box on
/workspace/search until the global search results settle, on a profile
with the M3U fixture and the mock's existing 12,000-item `large` Xtream
catalog. Counters: bridge calls and SQL statements per search (with a
per-keystroke breakdown), serial IPC depth, DOM mutations, change-detection
ticks, layout shift and long tasks; wall-clock last keystroke to settled
and first keystroke to first result.

Runs in the existing journeys target and the warn-only CI job, whose
summary now prints the per-keystroke table. Moves J3's picsum artwork
blocker into a shared helper.

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

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

@@ -72,5 +72,11 @@ runs:
                       (($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(" → "))", ""))
+                          "Serial IPC chain (first measured iteration): \(.chain | map("`\(.)`") | join(" → "))", ""),
+                      ([($j.iterations // [])[] | select(.warmup | not) | .evidence.perKeystroke // empty][0] // empty |
+                          "Per keystroke (first measured iteration):", "",
+                          "| Key | Query calls | Bridge calls | SQL statements | DOM mutations | CD ticks |",
+                          "| --- | ---: | ---: | ---: | ---: | ---: |",
+                          (.[] | "| `\(.key)` | \(.queryCalls) | \(.ipcCalls) | \(.sqlStatements) | \(.domMutations) | \(.cdTicks) |"),
+                          ""))
               ' "$SUMMARY" | tee -a "$GITHUB_STEP_SUMMARY"