Restore followup/DM reliability fixes wiped by 19:43Z tree-clean

Re-applies three workstreams lost when 793d3d7 committed over uncommitted
edits, reconciled against the parallel track's committed dm.py changes:
- followup-sweeper.py: backfill thread_uuid after successful nudge sends;
  record final_nudge_target=main on final-nudge routing (C1/C2)
- response-harvester.py: resolve followups on main-chat replies when
  final_nudge_target=main (C3); harvest ALL [RESULT] markers per message
- dm.py: pre-send placement gate (fail closed when post-nav URL lacks the
  target thread UUID; skips main) — purely additive over 793d3d7+f268d3d
- sidechat_manager.py: wait_for_chat_list() settle-poll for list population
  race (sidebar button renders before titles load)
- new: bin/tests/test_followup_fixes.py (25 tests), bin/placement-audit.py,
  bin/dm-log-taxonomy.py, bin/session-probe.py,
  docs/SIDECHAT-RELIABILITY.md, docs/UUID-ROTATION.md

Verified: 25/25 tests pass, py_compile clean, sweeper/harvester dry-runs clean.
Known limitation: gate catches wrong-placement, not wrong-mapping (false
autoprovision adopting the parked thread needs a creation check).

bin/sidechat_manager.py

@@ -4,6 +4,7 @@ Robust sidechat management for muse-chat-api.py.
 
 Provides:
 - ensure_sidebar(ws): Opens sidebar if closed, with retry
+- wait_for_chat_list(ws, name=None): Polls until sidebar list content loads
 - list_sidechats(ws): Returns list of side chat names, with retry
 - All operations logged for audit
 
@@ -56,13 +57,51 @@ def ensure_sidebar(ws, max_retries=3):
         return !!document.querySelector('[data-testid="hatch-chat-compose"]');
     })()""")
 
+def wait_for_chat_list(ws, name=None, timeout_s=15):
+    """Poll until the sidebar chat list content has loaded (and optionally
+    contains `name`, case-insensitive substring match).
+
+    Readiness requires at least one plausible chat-title line under the
+    "Side chats" header -- the header itself renders before items populate,
+    and the compose button that ensure_sidebar() keys on renders earlier
+    still. Clicking a chat title before the list is ready is a known
+    nav-miss contributor (the click lands nowhere and the SPA stays on /).
+
+    Per-poll CDP errors are treated as not-ready (browser may be mid-render
+    or mid-flap). Returns True when ready, False on timeout. Note: an
+    account with genuinely zero sidechats will always time out here.
+    """
+    import re as _re
+    want = (name or "").strip().lower()
+    deadline = time.time() + timeout_s
+    while time.time() < deadline:
+        try:
+            text = _ev(ws, "document.body.innerText") or ""
+        except Exception:
+            text = ""
+        idx = text.find("Side chats")
+        if idx != -1:
+            section = text[idx:idx + 2000].lower()
+            lines = [l.strip() for l in section.split("\n")]
+            titles = [l for l in lines[1:21]
+                      if 5 < len(l) < 80
+                      and not _re.fullmatch(r"\d+[mh]", l)
+                      and l != "unread updates"]
+            if titles and (not want or want in section):
+                return True
+        time.sleep(1)
+    return False
+
 def list_sidechats(ws):
     """
     List side chat names. Returns list of strings.
-    Ensures sidebar is open first.
+    Ensures sidebar is open first, then waits for the list content to
+    populate before scraping (the open signal fires before content loads).
     """
     if not ensure_sidebar(ws):
         return []
+    if not wait_for_chat_list(ws):
+        return []
 
     result = _ev(ws, """(() => {
         const text = document.body.innerText;