[Feat] (Pipe): land the P1 contract - PipeInputsSwitch.h with MGB_CTX, the 63-field PipeInputs block with type-identical accessors, FillPoints.def and its G5b generator, the MOBILEGL_PIPE_PUSH/VERIFY options and the three verify knobs; pull build unchanged

- MG_Pipe/PipeInputsSwitch.h is the strangler switch (ARCHITECTURE.md 9.2): MGB_CTX is the
  live GLContext in the pull build and &gPipeInputs under MOBILEGL_PIPE_PUSH, so the pull
  arm's pGLContext spelling stays outside MG_Backend/ and purity gate C's grep.
- MG_Backend/MGPipe/PipeInputs.h holds one struct with every accessor a backend reads (63:
  the 61 Coverage.def rows plus GetBoundTransformFeedbackLifetimeId and
  HasOpenTransformFeedbackSpan), each keeping its GLContext name, parameters and return
  type so the site conversion is type-neutral; V fields are copied values, O fields are
  SharedPtr copies or pointers into the context, the seven F fields forward to the live
  context from MG_Impl/Pipe/PipeFill.cpp and are the only sticky ones (Coverage.def's
  MGP_COVERAGE_STICKY_LIST argues each: argument-keyed lookups and reverse-channel writes,
  never a version or generation counter).
- MG_Pipe/FillPoints.def is the verb table: one row per GLFunctionsTable function pointer in
  declaration order (69), nine classes and the may-read field rows; gen_pipe.py parses the
  struct and refuses a row set that is not exactly its member set, then emits
  generated/PipeFillPoints.inc (verb enum, class tables, per-class field masks with the
  sticky fields OR'ed in). MGP_FILL(Verb) in MG_Impl/Pipe/PipeFill.h is the fill point;
  MGPipeFillForVerb only bumps the serial, records the verb and stamps the sticky fields
  here - the per-class copies land in the next commit, the fill points in MG_Impl after.
- MOBILEGL_PIPE_POISON is derived once in PipeInputs.h from MOBILEGL_PIPE_PUSH and the DEBUG
  level, MOBILEGL_BUILD_DISAGGREGATED or MOBILEGL_PIPE_VERIFY (the tree has no
  MOBILEGL_DEBUG); under it every accessor is a read-side freshness check that aborts with
  Fatal{UnmigratedPipeInput, "Field@Verb"}.
- CMake: MOBILEGL_PIPE_PUSH and MOBILEGL_PIPE_VERIFY options (VERIFY forces PUSH on), the two
  new sources appended only under PUSH, the compile definitions; Config.h/ConfigLoader.cpp
  gain PipeVerifyFatal / PipeVerifyCorrupt / PipePoisonOmit under #if MOBILEGL_PIPE_PUSH so
  the pull build's FeaturesTable does not change size.
- Pull build proof: symbol_report.py against the 087685d1 baseline reports 0 added / 0
  removed / 0 resized / 0 renamed and a .text delta of 0; ctest -N names unchanged; gen_pipe
  --check clean; unit tests green in the pull, push and verify builds.

MobileGL/ConfigLoader.cpp

@@ -244,6 +244,13 @@ namespace MobileGL::MG_ConfigLoader {
         // that starts with MOBILEGL_ is visible to these queries by construction.
         features.PipePush = QueryEnvUint64("MOBILEGL_PIPE_PUSH", 0);
         features.PipeVerify = QueryEnvFlag("MOBILEGL_PIPE_VERIFY");
+#if MOBILEGL_PIPE_PUSH
+        // Defaults ON: read as a tri-state so only an explicitly falsy value turns it off.
+        features.PipeVerifyFatal =
+            QueryEnvQuirkOverride("MOBILEGL_PIPE_VERIFY_FATAL") != MG_Config::QuirkOverride::ForceOff;
+        QueryEnvVariable("MOBILEGL_PIPE_VERIFY_CORRUPT", features.PipeVerifyCorrupt, "");
+        QueryEnvVariable("MOBILEGL_PIPE_POISON_OMIT", features.PipePoisonOmit, "");
+#endif
         features.PipeStats = QueryEnvFlag("MOBILEGL_PIPE_STATS");
         // Defaults ON, so the flag has to be read as a tri-state rather than as a plain
         // truthy check: unset must keep the memos, and only an explicitly falsy value may