[Feat] (TraceApp, CI): pass arbitrary env vars through the retrace lane

- PLAN-B.md §8.2 and appendix B add a batch of new runtime switches
  (MOBILEGL_PIPE_PUSH / _VERIFY / _STATS / _LEGACY_MEMOS / _TEXEL_RETAIN_MB /
  _INDEX_MIRROR_MB, plus MOBILEGL_IPC_* later), and §11 P0 wants them parsed beside
  the existing ones. Today every knob that has to reach an Android replay costs an
  edit in five files - run_android_retrace_local.py, trace-replay-ci.sh,
  TraceReplayActivity's request record, the JNI marshalling, and the setenv block in
  trace_replay_core.cpp. That per-knob tax is what this replaces: one extra,
  `--es mobilegl_env "K=V;K=V"`, carries all of them.
- Applied last, immediately before dlopen(libMobileGL.so), so it can also override
  the dedicated fields above it - MobileGL's config is read during the load, and an
  escape hatch that cannot beat the defaults is not one. An entry with no '=' unsets
  the variable, which is the only way to clear a default the marshalling sets.
- The existing per-knob flags stay: they carry semantics beyond a setenv (use_angle
  also selects a variant, the dump lists are joined, DirectVulkan forces the
  R11G11B10F fallback), and rewriting them as env strings would move that logic into
  the callers.
- Surface: --env / MOBILEGL_TRACE_ENV in trace-replay-ci.sh, repeatable --env
  KEY=VALUE in run_android_retrace_local.py, `mobilegl_env` intent extra,
  Request::envOverrides.
- The two-level parse now lives in trace_env_overrides.hpp, beside the semicolon
  splitter it shares with the texture and FBO dump lists, and
  tools/trace_replay/trace_env_overrides_test.cpp pins it: the empty entries a
  trailing ';' leaves behind must not become unsetenv(""), `K=` must stay a Set of
  the empty string rather than an Unset (a knob read with getenv() != nullptr sees
  those as opposite answers), and only the FIRST '=' may separate, or a value
  carrying '=' is truncated without a word of warning. The whole MOBILEGL_PIPE_*
  batch rides on this parse, and the only lane that exercised it end to end was an
  on-device retrace, which would have reported a splitting bug as "the knob had no
  effect".
- The check is built and RUN at build time and mobilegl_trace_replay depends on it,
  so `cmake --build ... --target mobilegl_trace_replay` - the exact command of
  test.yml's "Build trace replay" job, which never invokes ctest - runs it. It is
  assert-free on purpose: that lane configures Release, and <cassert> under NDEBUG
  would compile every check into a green run that checked nothing. Negative control:
  swapping find('=') for rfind('=') fails 2 checks, and keeping the splitter's empty
  entries fails 2 more.

tools/trace_replay/CMakeLists.txt

@@ -281,6 +281,25 @@ else()
             "-Wl,--end-group")
 endif()
 
+# The `K=V;K=V` environment passthrough is the only hand-rolled parse between a CI flag
+# and setenv(), and the lane that exercises it end to end is an on-device retrace, which
+# would report a splitting bug as "the knob had no effect". This pins it instead, in the
+# cheapest place that no configuration can skip: the check is built and RUN at build time,
+# and mobilegl_trace_replay depends on it, so `cmake --build ... --target
+# mobilegl_trace_replay` - the exact command of test.yml's "Build trace replay" - runs it
+# even though that job never invokes ctest.
+if(NOT CMAKE_CROSSCOMPILING)
+    add_executable(mobilegl_trace_env_overrides_test
+            "${MOBILEGL_TRACE_ROOT}/trace_env_overrides_test.cpp")
+    target_compile_features(mobilegl_trace_env_overrides_test PRIVATE cxx_std_17)
+    target_include_directories(mobilegl_trace_env_overrides_test PRIVATE
+            "${MOBILEGL_TRACE_SHARED_CPP_DIR}")
+    add_custom_command(TARGET mobilegl_trace_env_overrides_test POST_BUILD
+            COMMAND mobilegl_trace_env_overrides_test
+            COMMENT "Checking the retrace env passthrough parser")
+    add_dependencies(mobilegl_trace_replay mobilegl_trace_env_overrides_test)
+endif()
+
 if(MOBILEGL_TRACE_REPLAY_MOBILEGL_LIBRARY)
     set(mobilegl_trace_replay_mobilegl_library "${MOBILEGL_TRACE_REPLAY_MOBILEGL_LIBRARY}")
 else()