fix: replace CSQLite systemLibrary with proper C target for Android/Linux

Converts CSQLite from a .systemLibrary (which required an absolute path
to sqlite3.h and relied on pkg-config) to a proper C .target that uses
an umbrella header with '#include <sqlite3.h>' (angle-bracket search).

Why this works everywhere:
- macOS:   CSQLite not used — Swift imports SQLite3 framework directly
- Linux:   '#include <sqlite3.h>' resolves via system include path
           (libsqlite3-dev installs /usr/include/sqlite3.h)
- Android: '--swift-sdk aarch64-unknown-linux-android24' sets NDK sysroot;
           sqlite3.h is a public NDK API since API 5, found at
           $(SYSROOT)/usr/include/sqlite3.h automatically

The old .systemLibrary approach failed on Android because:
  - pkg-config is not available in the cross-compilation environment
  - The absolute path '/usr/include/sqlite3.h' does not exist in the sysroot
  - The relative path 'sqlite3.h' was not found without pkg-config flags

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: vkuttyp

Package.swift

@@ -7,9 +7,16 @@ let sqliteSystemLibTargets: [Target] = []
 let sqliteNioExtraDeps:       [Target.Dependency] = []
 let sqliteNioLinkerSettings:  [LinkerSetting] = [.linkedLibrary("sqlite3")]
 #else
+// On Linux and Android cross-compilation:
+//   CSQLite.h does `#include <sqlite3.h>` (angle brackets → system search path).
+//   Linux:   resolves to /usr/include/sqlite3.h  (with libsqlite3-dev)
+//   Android: resolves to $(NDK_SYSROOT)/usr/include/sqlite3.h (NDK public API since API 5)
 let sqliteSystemLibTargets: [Target] = [
-    .systemLibrary(name: "CSQLite", pkgConfig: "sqlite3",
-                   providers: [.apt(["libsqlite3-dev"])]),
+    .target(
+        name: "CSQLite",
+        publicHeadersPath: "include",
+        linkerSettings: [.linkedLibrary("sqlite3")]
+    ),
 ]
 let sqliteNioExtraDeps:       [Target.Dependency] = [.target(name: "CSQLite")]
 let sqliteNioLinkerSettings:  [LinkerSetting] = []

Sources/CSQLite/include/CSQLite.h

@@ -0,0 +1,5 @@
+#pragma once
+// Umbrella header: forwards to the system SQLite3 header.
+// On Linux (libsqlite3-dev) this resolves to /usr/include/sqlite3.h.
+// On Android (NDK sysroot) this resolves to $(SYSROOT)/usr/include/sqlite3.h.
+#include <sqlite3.h>

Sources/CSQLite/include/module.modulemap

@@ -0,0 +1,5 @@
+module CSQLite {
+    umbrella header "CSQLite.h"
+    link "sqlite3"
+    export *
+}

Sources/CSQLite/module.modulemap

@@ -0,0 +1 @@
+// Empty shim — required so Swift Package Manager recognises this as a C target.