This pull request introduces significant improvements and documentation to the kernel, focusing on enhanced task management, scheduler functionality, and core kernel data structures. It adds comprehensive documentation for kernel primitives, data structures, scheduler, and timing helpers, introduces new assembly routines for task sleeping and context management, and updates task and context structures to support more robust scheduling and blocking mechanisms. Additionally, linker and configuration changes expand available memory and feature flags.

Key changes include:

Documentation Additions and Improvements

• Added new documentation files detailing kernel data structures, primitives, scheduler, and timing helpers, including invariants and usage guidelines (Documentation/kernel/data_structure.md, Documentation/kernel/primitives.md, Documentation/kernel/scheduler.md, Documentation/kernel/timing_helpers.md). [1] [2] [3] [4]
• Expanded task documentation with details on the idle task and updated task/block control structures (Documentation/kernel/task.md). [1] [2] [3] [4]

Task and Scheduler Enhancements

• Introduced TaskBlockControl enum and integrated it into the Task struct to better represent blocked task reasons and states (Documentation/kernel/task.md).
• Added support for an idle task to ensure the scheduler always has at least one runnable task (Documentation/kernel/task.md, Cargo.toml). [1] [2]
• Improved TaskContext struct to include new fields (ra, mstatus) for more accurate context switching and status tracking (Documentation/kernel/task.md, src/arch/riscv32/task/task_context.rs). [1] [2]
• Updated context initialization to set mstatus appropriately for interrupt enabling (src/arch/riscv32/task/task_context.rs).

Assembly and Context Switch Improvements

• Added new assembly routines for sleeping (sleep.S), improved context save/restore (save_context.S, restore_context.S), and updated trap entry/exit logic for preemptive scheduling and correct status management (src/arch/riscv32/asm/sleep.S, src/arch/riscv32/asm/save_context.S, src/arch/riscv32/asm/restore_context.S, src/arch/riscv32/asm/trap_entry.S). [1] [2] [3] [4] [5]
• Modified yield and scheduler invocation to streamline rescheduling (src/arch/riscv32/asm/yield.S).
• Registered new assembly routines in the architecture module (src/arch/riscv32/asm/mod.rs, src/arch/riscv32/mod.rs). [1] [2]

Feature Flags and Build Configuration

• Added new feature flags for kernel logs, early print, test mode, and idle task support in Cargo.toml.
• Increased RAM and ROM size in linker script to accommodate larger kernel and features (linkers/linker_test_mode.ld).
• Updated crate version to 0.4.5 (Cargo.toml).

Utilities and Helpers

• Added a helper function to retrieve the current CPU core using the mhartid CSR (src/arch/riscv32/helpers.rs).

These changes collectively improve the kernel's robustness, documentation, and scheduling capabilities, making it easier to maintain, extend, and understand.