valgrind: broaden shared_ptr control-block suppression to whole _Sp_counted_base

A fresh drd run (4fb73d5) surfaced a new benign ConflictingAccess in
webserver_register_path_prefix: the MHD worker dropping the last shared_ptr ref
to a resource during modded_request::~modded_request in request_completed, while
the main thread unregistered under route_cache's std::mutex. The access is the
libstdc++ control-block teardown (~_Sp_counted_base / _M_weak_release), atomic
refcount (_Lock_policy _S_atomic) which drd/helgrind cannot model.

This is the same benign class the existing _M_release entry covered, but the
teardown surfaces through different control-block methods each run depending on
thread timing, so a per-method match plays whack-a-mole. Broaden the match to
the whole _Sp_counted_base family (dtor, _M_release, _M_weak_release,
_M_dispose) in both supp files. Pure libstdc++ frame — a real libhttpserver
locking race surfaces a libhttpserver top frame, never _Sp_counted_base; TSan,
which models the atomic refcount, is green on these tests. Mirrors the existing
broad-third-party entries (basic_string, vector, _Rb_tree, _Sp_counted_ptr_inplace).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NpysYDDJac63yz2mZKKiDf

Author: etr

test/valgrind-drd.supp

@@ -73,13 +73,22 @@
    fun:thread_main_handle_connection
 }
 
-# --- libstdc++ shared_ptr control-block refcount ----------------------------
-# _M_release decrements an atomic use-count; DRD does not model the libstdc++
-# atomic and reports a benign counter conflict.
-{
-   libstdcxx-shared-ptr-refcount-release
-   drd:ConflictingAccess
-   fun:*_Sp_counted_base*_M_release*
+# --- libstdc++ shared_ptr control-block (atomic refcount + teardown) ---------
+# The shared_ptr control block (_Sp_counted_base and derived) keeps an atomic
+# use-/weak-count DRD does not model, so it reports a benign conflict whenever
+# the last ref is dropped on one thread while another touched the block. The
+# teardown surfaces through many control-block methods depending on timing
+# (_M_release, _M_weak_release, _M_dispose, ~_Sp_counted_base), so match the
+# whole _Sp_counted_base family rather than a single method -- otherwise each
+# timing-dependent valgrind run hits a different frame (observed: the MHD worker
+# dropping the last resource ref in modded_request::~modded_request during
+# request_completed while the main thread unregisters). Pure libstdc++ frame; a
+# real libhttpserver locking race surfaces a libhttpserver top frame. TSan is
+# green on the same tests.
+{
+   libstdcxx-shared-ptr-control-block
+   drd:ConflictingAccess
+   fun:*_Sp_counted_base*
 }
 
 # --- libstdc++ atomic<shared_ptr> free-function ops (_Sp_locker spinlock) -----

test/valgrind-helgrind.supp

@@ -70,13 +70,23 @@
    fun:thread_main_handle_connection
 }
 
-# --- libstdc++ shared_ptr control-block refcount ----------------------------
-# _M_release decrements an atomic use-count (_Lock_policy _S_atomic); Helgrind
-# does not model the libstdc++ atomic and reports a benign counter race.
+# --- libstdc++ shared_ptr control-block (atomic refcount + teardown) ---------
+# The shared_ptr control block (_Sp_counted_base and its _Sp_counted_ptr_inplace
+# derived) maintains an atomic use-/weak-count (_Lock_policy _S_atomic) that
+# Helgrind does not model, so it reports a benign race whenever the last ref is
+# dropped on one thread while another touched the block. The teardown surfaces
+# through many control-block methods depending on timing -- _M_release,
+# _M_weak_release, _M_dispose, ~_Sp_counted_base -- so match the whole
+# _Sp_counted_base family rather than a single method (each valgrind run
+# otherwise hits a different frame; observed the worker dropping the last
+# resource ref in modded_request::~modded_request during request_completed while
+# the main thread unregisters). Pure libstdc++ frame: a real libhttpserver
+# locking race surfaces a libhttpserver top frame, never _Sp_counted_base. TSan,
+# which models the atomic refcount, is green on the same tests.
 {
-   libstdcxx-shared-ptr-refcount-release
+   libstdcxx-shared-ptr-control-block
    Helgrind:Race
-   fun:*_Sp_counted_base*_M_release*
+   fun:*_Sp_counted_base*
 }
 
 # --- libstdc++ atomic<shared_ptr> free-function ops (_Sp_locker spinlock) -----