fix(fase1): spike B fan-out measures real conversation bleed, not a counter

/trekreview flagged the Spike B(b) fan-out experiment as BROKEN_SUCCESS_CRITERION
(BLOCKER): it asserted a per-client call_count reached 3 on a reused instance vs
1 on a fresh one — a tautology true for any un-reset mutable counter, independent
of MAF, that never exercised the real G2/B7 shared-Workflow state-corruption
footgun. It was a false-confirm of a de-risk assumption.

Rebuilt to observe genuine MAF thread state via the messages each participant
RECEIVES (new FakeChatClient.received_texts seam):
- shared_instance_conversation_bleed: a reused built ConcurrentBuilder Workflow
  accumulates the conversation across .run() calls — run N's participants receive
  runs 0..N-1's prompts/replies (measured [[p0],[p0,p1],[p0,p1,p2]], strictly
  monotonic) => genuine cross-run contamination.
- fresh_instance_conversation_isolation: a fresh instance per run gives each a
  clean thread => each participant sees only its own project ([[p0],[p1],[p2]]).

Assumption now CONFIRMED with a meaningful observable. findings-b.md gains a
Method note recording why it was rebuilt; README rows updated.

Also fixes the MINOR: a_groupchat.run_live now mkdirs the findings dir before
write_text so a post-disposal run does not lose the measured result.

Gate green: ruff check + format, mypy src, pytest 48 passed / 1 skipped.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fif1r1En5W542HbZV88yMH

spikes/a_groupchat.py

@@ -176,6 +176,10 @@ async def run_live(
 
     table = render_comparison(mc, single)
     if write_findings:
+        # Guard the parent dir: the spike may run after the documented disposal
+        # (`rm -rf docs/fase1-spikes`), and a missing dir must not discard the
+        # just-measured live result with a bare FileNotFoundError.
+        _FINDINGS.parent.mkdir(parents=True, exist_ok=True)
         _FINDINGS.write_text(_findings_doc(table), encoding="utf-8")
     return {"mc": mc, "single": single, "verdict": verdict(mc, single), "table": table}