[Perf] (MG_State): dedupe shader compiles by source hash

Iris-style packs hand MobileGL the same source text repeatedly: probed across
three shaderpack traces, 28-32% of all glCompileShader work was redundant -
~9% same-object recompiles with byte-identical source, ~21% distinct shader
objects sharing identical source (the same common GLSL chunk glued into many
program stages). Two layers, both keyed by XXH64 + length with a full byte
compare on every hit (correctness never rides on the hash):

- Per-object: a successful (or failed) compile remembers its source hash;
  glShaderSource with byte-identical text keeps the compiled state and
  glCompileShader on unchanged source returns immediately. Deterministic
  (stage, source) pipeline makes the memo observationally identical to
  recompiling; the consume-once TakeShaderForLink re-parse path is untouched.

- Cross-object: a per-context bounded cache (ProgramState-owned, declared to
  outlive every shader object) shares the preprocessed source, both explicit
  side-channel maps, and the validation verdicts between objects with equal
  source; only the glslang parse stays per-object. Single-GL-thread today;
  flagged for a mutex when compiles go async (P1).

Interleaved A/B on the iterationrp trace (the recompile-heavy pack):
5.65s -> 5.46s median total replay, every round faster; BSL/complementary
stay flat (their duplicate sources are the small common shaders, so calls
drop but wall time is parse-bound on unique sources). Full DirectGLES
retrace, 445-test unit suite, and dedupe-semantics tests (no-op recompile,
invalidation on new source, failed-compile memo, cache bounds) all green.

CMakeLists.txt

@@ -291,6 +291,7 @@ set(SOURCE_FILES
     MobileGL/MG_State/GLState/TextureState/TextureState.cpp
     MobileGL/MG_State/GLState/ProgramState/ProgramObject.cpp
     MobileGL/MG_State/GLState/ProgramState/ShaderObject.cpp
+    MobileGL/MG_State/GLState/ProgramState/ShaderPreprocessCache.cpp
     MobileGL/MG_State/GLState/ProgramState/ProgramState.cpp
     MobileGL/MG_State/GLState/RenderState/RenderState.cpp
     MobileGL/MG_State/GLState/FramebufferState/FramebufferObject.cpp