fix(e2e): isolate coverage-pass WORK dir; filter chain-only e2e report section

The final whole-branch review (post Task 10) found two real cross-task
integration bugs:

- run_e2e.sh's coverage-instrumented scenario re-run only rebound OUT_DIR
  in its subshell, not WORK. proc_perf.py/plots.py write perf_*.json and
  wave_*.png into WORK, so every --cpp-coverage run (the default) silently
  clobbered the primary pass's perf/waveform data with the instrumented
  re-run's numbers before report_build.py read them -- defeating the
  "uncontaminated performance metrics" goal of the coverage-double-run
  design. Fixed by rebinding WORK the same way OUT_DIR already was.

- report_build.py's build_e2e() iterated all results["scenarios"] with no
  kind filter, so the direct/recorder/debug/tcplogger scenarios (already
  covered by their own dedicated report sections since Task 8) also leaked
  into the chain-only Scenarios/Performance sections and headline e2e
  pass/fail count as degenerate rows. Fixed by filtering to kind=="chain".

Verified end-to-end with a full ./run_e2e.sh run: coverage pass now writes
to /tmp/chain_e2e/coverage_pass/ (confirmed via log), and report_data.json's
e2e section now reports 51 (chain-only) scenarios instead of all 56.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Test/E2E/suite/report_build.py

@@ -74,6 +74,8 @@ def build_e2e(results, work):
     scen = []
     corrs, rss_vals, cpu_vals, tput_vals = [], [], [], []
     for r in results.get("scenarios", []):
+        if r.get("kind", "chain") != "chain":
+            continue
         sid = r["id"]
         metrics = r.get("metrics", {})
         sigs = []