feat: refactor user extension handling and update MPW generation

- Updated README.md to clarify the status of KianV RV32IMA and PicoRV32 in the user-extension map.
- Modified Makefile to include USER_EXTENSIONS_SOURCE in MPW variant stamp dependencies.
- Enhanced integration README.md to better describe the relationship between user_extensions.json and mpw.toml.
- Updated generate_user_extensions.py to include design_id in ExtensionTarget and enforce unique design IDs.
- Changed user_extensions.json schema version to 2 and added design_id fields for core and IP targets.
- Refactored generate_mpw.py to support the new manifest structure and validate design selections.
- Adjusted sysctrl_tb.sv test cases to reflect changes in user core reset states.
- Enhanced test_opipsram.py to utilize ccache for improved compilation performance.
- Updated test_script_tools.py to include new tests for active manifest generation.
- Modified test_user_extensions.py to validate isolated scalar bindings and reject duplicate design selections.

tests/rtl/sysctrl_tb.sv

@@ -155,20 +155,20 @@ module sysctrl_tb;
     if (read_data !== 32'd151) $fatal(1, "USB2 performance snapshot was not recorded");
 
     read_register(32'h1000_B020, read_data);
-    if (read_data !== 32'h0000_003F) $fatal(1, "user cores were not held in reset");
-    write_register(32'h1000_B000, 32'h0000_0006);
+    if (read_data !== 32'h0000_000F) $fatal(1, "user cores were not held in reset");
+    write_register(32'h1000_B000, 32'h0000_0004);
     read_register(32'h1000_B024, read_data);
     if ((read_data & 32'h0000_083F) !== 32'h0000_0800) begin
       $fatal(1, "out-of-range user core selection was accepted");
     end
     write_register(32'h1000_B024, 32'h0000_0800);
     write_register(32'h1000_B000, 32'h0000_0001);
-    write_register(32'h1000_B020, 32'h0000_003D);
+    write_register(32'h1000_B020, 32'h0000_000D);
     read_register(32'h1000_B024, read_data);
     if ((read_data & 32'h0000_0300) !== 32'h0000_0300 || (read_data & 32'h1F) != 1) begin
       $fatal(1, "user core start state was not recorded");
     end
-    write_register(32'h1000_B020, 32'h0000_003F);
+    write_register(32'h1000_B020, 32'h0000_000F);
     read_register(32'h1000_B024, read_data);
     if ((read_data & 32'h0000_0300) !== 32'h0000_0200) begin
       $fatal(1, "user core stop state was not recorded");