kernel: Add MONITOR/MWAIT C1 idle support (P24)

Implement CPU power-saving idle loop using x86 MONITOR/MWAIT:
- Add monitor(), mwait(), enable_and_mwait() to interrupt module
- Detect MWAIT availability via CPUID at boot
- Use MONITOR+MWAIT instead of STI+HLT when supported
- Expose /scheme/sys/cstate_policy for userspace control
- Add RdWr Kind variant to sys scheme for read+write files

recipes/core/kernel/recipe.toml

@@ -45,6 +45,8 @@ patches = [
     "../../../local/patches/kernel/P22-x2apic-madt-fallback.patch",
     # P23: sys:msr scheme — kernel MSR read/write via /scheme/sys/msr/<cpu>/<msr>
     "../../../local/patches/kernel/P23-sys-msr-scheme.patch",
+    # P24: C-state idle loop with MONITOR/MWAIT support
+    "../../../local/patches/kernel/P24-cstate-mwait-idle.patch",
 ]
 
 [build]