Update README. Fix source tests.

[vshcryabets]

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[Copilot]

Pull Request Overview

This PR implements the isPullSource() and isPushSource() methods for TestSourceYUV420 and upgrades the build infrastructure to CMake 3.31. The key changes include adding native JNI implementations for source type checking, updating CMake configuration with explicit linker options for symbol inclusion, and adding comprehensive test coverage.

• Implements isPullSource() and isPushSource() methods in TestSourceYUV420 with JNI bindings
• Upgrades CMake minimum version to 3.31 with platform-specific linker configurations
• Adds tests for pull/push source type detection and improves test initialization

Reviewed Changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
linux/libs/jnilib/CMakeLists.txt	Updates CMake version to 3.31 and adds platform-specific linker flags for forcing symbol inclusion
linux/apps/gtk_preview/CMakeLists.txt	Removes commented-out code reference
libausbc/src/test/java/com/vsh/source/TestSourceYUV420Tests.kt	Adds tests for isPullSource() and isPushSource(), ensures source is opened before testing
libausbc/src/test/java/com/vsh/source/PullToPushSourceTests.kt	Refactors test to use real TestSourceYUV420 instead of anonymous mock object and fixes parameter name
libausbc/src/main/jni/modules/sources/src/jni/JniTestSource.cpp	Adds native implementations for isPullSource() and isPushSource(), removes commented registration code
libausbc/src/main/jni/modules/sources/src/jni/JniSources.cpp	Removes call to deleted JniTestSource_register() function
libausbc/src/main/jni/modules/sources/include/jni/JniTestSource.h	Removes declaration of deleted JniTestSource_register() function
libausbc/src/main/java/com/vsh/source/TestSourceYUV420.kt	Implements isPullSource() and isPushSource() with JNI calls and adds native method declarations
libausbc/src/main/java/com/vsh/source/PullToPushSource.kt	Adds error message to orElseThrow() for better diagnostics
libausbc/build.gradle.kts	Updates CMake configuration syntax and adds version specification
libausbc/CMakeLists.txt	Updates minimum required CMake version to 3.31
gradle/libs.versions.toml	Adds CMake version 3.31.6 to version catalog
README.md	Adds build instructions with CMake and NDK version requirements

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