Compile Git for host and Android target platforms

- Rename CrossCompileGitForAndroid.sh to CompileGitForAllTargetPlatforms.sh and support host, Android-only, and all-target modes.
- Compile a development-host Git binary at build/host-git/libgit.so for JVM tests.
- Route level and sandbox tests through GITHUG_TEST_GIT_BINARY so they exercise the compiled host Git rather than the system Git fallback.
- Cross-compile packaged Git binaries for arm64-v8a, armeabi-v7a, x86, and x86_64 Android targets.
- Remove the temporary device-bound Pull rebase instrumentation path from the previous debugging attempt.

app/src/test/java/solutions/tretter/githugandroid/LevelSolutionsTest.kt

@@ -31,7 +31,7 @@ class LevelSolutionsTest {
     @Test
     fun knownSolutionsCompleteEveryLevel() {
         val evidence = StringBuilder()
-        val gitBinary = systemGitBinary()
+        val gitBinary = testGitBinary()
         val runtimeRoot = testSandboxRoot().apply {
             deleteRecursively()
             mkdirs()
@@ -120,7 +120,16 @@ class LevelSolutionsTest {
             return File(appDir, "build/test-sandboxes/level-solutions")
         }
 
-        fun systemGitBinary(): File {
+        fun testGitBinary(): File {
+            System.getenv("GITHUG_TEST_GIT_BINARY")
+                ?.takeIf { it.isNotBlank() }
+                ?.let { File(it) }
+                ?.takeIf { it.exists() && it.canExecute() }
+                ?.let { return it }
+
+            val repoHostGit = File(repoRoot(), "build/host-git/libgit.so")
+            if (repoHostGit.exists() && repoHostGit.canExecute()) return repoHostGit
+
             val candidates = listOf(
                 File("/usr/bin/git"),
                 File("/usr/local/bin/git"),
@@ -136,6 +145,11 @@ class LevelSolutionsTest {
             return File(output)
         }
 
+        fun repoRoot(): File {
+            val userDir = File(System.getProperty("user.dir") ?: ".")
+            return if (File(userDir, "app/build.gradle.kts").exists()) userDir else userDir.parentFile ?: userDir
+        }
+
         fun RepoState.describeForEvidence(): String = buildString {
             appendLine("initialized=$initialized")
             appendLine("headBranch=$headBranch")