Refactor ShaderRenderTester::validate (#2825)

Refactor the monolithic validate() into smaller methods:
- loadOptions(): parse test suite options and check target
- collectTestFiles(): gather .mtlx files with override filtering
- loadDependentLibraries(): load library dependencies once
- initializeGeneratorContext(): set up renderer, CMS, units, GenContext
- loadAndValidateDocument(): per-file load, validate, find renderables

Add helper service classes for instrumentation:
- TestRunLogger: file logging and stream management
- TestRunProfiler: manages profiling timers and summary output
- TestRunTracer: wraps Perfetto tracing behind pimpl (conditional build)

Author: ashwinbhat