[Feat] (Metrics): land the MGPipe boundary counters - bytes, dynamic accessor calls and the six memo gates, off unless MOBILEGL_PIPE_STATS is set

- Plan B section 11 P0 asks for TracyPlot per-frame counters on both sides of the
  boundary, and section 2.3.1 adds the deliverable v1 did not have: DYNAMIC call
  counters. The static call-site counts everyone quotes (Espryt 124 / Magma 169) are
  not the per-draw cost, because every one of those paths is memo-gated; without a
  dynamic counter P2's verdict stays a guess. The tree had no per-frame byte or call
  measurement at all - MG_Util/Metrics is format arithmetic, and Tracy has zones but
  no plots.
- Cost when off: g_pipeStatsEnabled is a plain global Bool latched once in
  Initialize() right after MG_ConfigLoader::Init(), and every counting site is
  `if (PipeStats::Enabled()) ...` - one load of a hot global and one never-taken
  branch. The counters are relaxed atomics because buffer and texture staging are
  reachable from more than one thread; the off path never touches them.
- Eight byte classes (stage-buffer, stage-texture, stage-ubo-global, stage-ubo-named,
  stage-vertex-client, stage-index-client, persistent-map-push and the
  residual-value-block placeholder of section 6.3), six call classes and the six memo
  gates of section 2.3.1, each as a hit/miss pair. Names are minted now, including the
  two that stay 0 in P0, so no recorded baseline is invalidated by a later rename -
  which is why the name set is pinned by a test.
- Reporting: TracyPlot per counter per frame when TRACY_ENABLE; with
  MOBILEGL_PIPE_STATS=1 one MGLOG_I summary line every 120 frames and at teardown, plus
  a JSON dump to MOBILEGL_PIPE_STATS_FILE when that is set. MGLOG_I against the usual
  "MGLOG_D for non-critical" rule on purpose: the line has to survive an INFO build -
  the only build a device runs - it is at most one line per 120 frames, and it exists
  only when the operator asked for it.
- Summary lines report disjoint WINDOWS, not run totals: a run total over a workload
  that changes shape (load, then steady state) averages away the very number section
  2.3.1 wants an absolute value for.
- MOBILEGL_PIPE_STATS_FILE joins the six switches parsed in e48a3582; like them it
  needs no allow-list entry because InitializeAcceptedEnvVariables accepts every
  MOBILEGL_-prefixed variable by construction.

CMakeLists.txt

@@ -238,6 +238,8 @@ set(SOURCE_FILES
 
     MobileGL/MG_Util/Metrics/BufferMetrics.cpp
 
+    MobileGL/MG_Util/Metrics/PipeStats.cpp
+
     MobileGL/MG_Util/Converters/GLToStr/GLEnumConverter.cpp
     MobileGL/MG_Util/Converters/EGLToStr/EGLEnumConverter.cpp
     MobileGL/MG_Util/Converters/MGToStr/DataTypeConverter.cpp