refactor(boot): 收敛 RISC-V 和 LoongArch 主核启动流程

[ZIYAN137]

变更内容

本 PR 处理 #249，主要收敛 RISC-V 和 LoongArch 中重复的主核启动流程。

• 在 kernel::boot 中新增 PrimaryBootOps 和 run_primary_boot()，统一主核启动顺序
• RISC-V / LoongArch boot 入口只保留架构相关 hook
• 新增 arch::interrupts_enabled()，移除 kernel/boot.rs 对 arch::intr 的直接依赖
• 新增 arch::kernel::task::init_kernel_trap_frame()，避免 common boot/task 代码直接构造 TrapFrame
• 修复 LoongArch timer CSR 配置问题
• 修复 LoongArch 首次调度 kernel entry 时的恢复路径

验证情况

• cargo fmt --check
• cargo check
• cargo check --target loongarch64-unknown-none
• cargo build
• cargo build --target loongarch64-unknown-none
• kernel/boot.rs 中已无直接 TrapFrame / arch::intr 引用
• QEMU smoke test：
  • RISC-V 可启动到 BusyBox
  • LoongArch 可进入 PID 1 并执行 /sbin/init 路径

Closes #249

[gemini-code-assist]

Code Review

This pull request unifies the primary boot sequence across architectures by introducing a generic run_primary_boot function and a PrimaryBootOps structure. It also standardizes kernel task initialization through the new init_kernel_trap_frame function and adds an interrupts_enabled method to the CpuOps trait. Review feedback identified critical copy-paste errors in the LoongArch implementation where RISC-V specific constants (SSTATUS_SIE) were used instead of the correct LoongArch CSR bits (CSR_CRMD_IE) for interrupt status checks.