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.

configs/README.md

@@ -3,8 +3,8 @@
 This directory contains committed Make configuration profiles. Each profile
 selects SoC, PDK, ISA, CSR support, application, linker layout, and the
 supported validation tier. The management core is fixed to Hazard3 with its
-JTAG Debug Module enabled. The user-extension fabric exposes stable C0-C5
-slots, with PicoRV32 at C5.
+JTAG Debug Module enabled. The user-extension fabric exposes C0-C3 slots for
+KianV RV32I, SERV, FemtoRV32, and DarkRISCV.
 
 `ci/` contains pull-request profiles, `cluster/` contains configurations
 requiring site tools or PDKs, and `benchmark/` contains fixed-workload baseline