POSIX Compat

Replace exception-handler context switch with two-phase kill path. Why it matters: Context-switching inside an exception handler violates the interrupt-handler contract and can corrupt kernel state. Focus on `KILL_PENDING` and `sys_exit(-11)` in `kernel/src/arch/x86_64/interrupts.rs` while you trace the snippet.

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Free old address space on execve success. Why it matters: Without freeing the old page tables, every execve leaks the entire previous address space. Focus on `free_process_page_table` and `free_process_page_table(cr3_phys)` in `kernel/src/mm/mod.rs` while you trace the snippet.

/// Free all user-space page table frames for the given PML4 physical address.
///
/// Walks PML4 indices 0–255 (user half), frees every mapped user-accessible
/// physical frame, and frees the page-table structure frames themselves.
///
/// Frame reclamation is a stub in the bump allocator (no-op); the real benefit
/// today is correctness — the function documents the ownership transfer and
/// will become fully effective once Phase 13 adds a free list.
///
/// # Safety
///
/// `cr3_phys` must be the physical address of a valid, now-unreachable PML4
/// that is no longer loaded in CR3. No other code may access the page table
/// after this call.
#[allow(dead_code)]
pub fn free_process_page_table(cr3_phys: u64) {
    use x86_64::structures::paging::{PageTable, PageTableFlags};
    let phys_off = VirtAddr::new(phys_offset());
    // SAFETY: cr3_phys is a valid PML4 frame being freed. The caller guarantees
    // it is no longer active (not in CR3) and has exclusive ownership.
    unsafe {
        let pml4: &PageTable = &*(phys_off + cr3_phys).as_ptr::<PageTable>();
        for p4 in 0usize..256 {
            let p4e = &pml4[p4];

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Reap dead kernel tasks from scheduler. Why it matters: Without reaping, dead tasks accumulate in the scheduler queue indefinitely. Focus on `TaskState::Dead` in `kernel/src/task/scheduler.rs` while you trace the snippet.

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