[MG_Backend] (P7): when the orphaned wire buffer stores still parked exceed MOBILEGL_IPC_WIRE_DEFERRED_MB (64 by default), the server takes the host-access sync point mid-frame and reclaims them all

The defer-path sweep frees an orphan once its last submission has retired, but inside one long
frame a store that was drawn from is named by recorded, unsubmitted work, and nothing submits it
until the frame ends. The knob is read once through MG_Config::Ipc like its siblings; above it
the park calls WaitForSubmitIndex(GetSyncPointSubmitIndex(), flush) - the call
WaitForWireBufferHostAccess makes - and sweeps again, which empties the list because no entry can
be tagged past the sync point just waited out. A sync point that cannot complete dies through
the existing Fatal{ResourceUnavailable, "buffer-write-sync"} site, now one helper shared with the
host write (fatal_census: 79 abort sites, unchanged). 0 is the negative control.

MobileGL/ConfigLoader.cpp

@@ -392,6 +392,9 @@ namespace MobileGL::MG_ConfigLoader {
         // are sanity, not policy.
         ipc.RingMb = QueryEnvUint32("MOBILEGL_IPC_RING_MB", 8, 1, 1024);
         ipc.StageMb = QueryEnvUint32("MOBILEGL_IPC_STAGE_MB", 32, 1, 4096);
+        // P7 wave 4 M2 (Config.h has the semantics). 0 is admitted ON PURPOSE as the negative
+        // control: no forced sync, and MagmaWireReclaimScenario's watermark case must go red.
+        ipc.WireDeferredMb = QueryEnvUint32("MOBILEGL_IPC_WIRE_DEFERRED_MB", 64, 0, 65536);
         ipc.SpinUs = QueryEnvUint32("MOBILEGL_IPC_SPIN_US", 50, 0, 1000000);
         // 0 is admitted ON PURPOSE and is the negative control of exit gate E3(a): it turns
         // the persistent-map push OFF, and PersistentCoherentMapScenario must go red.
@@ -462,10 +465,10 @@ namespace MobileGL::MG_ConfigLoader {
         if (MG_Config::Transport == MG_Config::TransportMode::Monolith) return;
         // One line, on the arm where these numbers decide behaviour, because every one of
         // them is a number a bug report has to quote.
-        MGLOG_I("Config: IPC ring=%uMiB stage=%uMiB spin=%uus persistent-block=%uKiB "
+        MGLOG_I("Config: IPC ring=%uMiB stage=%uMiB wire-deferred=%uMiB spin=%uus persistent-block=%uKiB "
                 "adopt-tier=%u verb-barrier=%u run-ahead=%u present-credit=%u strict=%d "
                 "audit=%d role-split-state=%d affinity='%s'",
-                ipc.RingMb, ipc.StageMb, ipc.SpinUs, ipc.PersistentBlockKb, ipc.AdoptTier,
+                ipc.RingMb, ipc.StageMb, ipc.WireDeferredMb, ipc.SpinUs, ipc.PersistentBlockKb, ipc.AdoptTier,
                 ipc.VerbBarrier, ipc.RunAhead, ipc.PresentCredit,
                 static_cast<int>(ipc.StrictErrors), static_cast<int>(ipc.Audit),
                 static_cast<int>(ipc.RoleSplitState), ipc.ServerAffinity.c_str());
@@ -474,6 +477,11 @@ namespace MobileGL::MG_ConfigLoader {
                     "expected to fail: the client still pulls 31 of 63 PipeInputs fields from a "
                     "live GLContext, so an unbarriered queue lets the server read future values");
         }
+        if (ipc.WireDeferredMb == 0) {
+            MGLOG_W("Config: MOBILEGL_IPC_WIRE_DEFERRED_MB=0 is the M2 NEGATIVE CONTROL: the "
+                    "server never forces a sync for orphaned wire buffer stores, so a long frame "
+                    "that respecifies and draws holds every one of them until the frame ends");
+        }
         if (ipc.PersistentBlockKb == 0) {
             MGLOG_W("Config: MOBILEGL_IPC_PERSISTENT_BLOCK_KB=0 is the E3(a) NEGATIVE CONTROL: "
                     "the persistent-map push is OFF and a coherent-map scenario must go red");