feat: P0-P6 kernel scheduler + relibc threading comprehensive implementation

P0-P2: Barrier SMP, sigmask/pthread_kill races, robust mutexes, RT scheduling, POSIX sched API
P3: PerCpuSched struct, per-CPU wiring, work stealing, load balancing, initial placement
P4: 64-shard futex table, REQUEUE, PI futexes (LOCK_PI/UNLOCK_PI/TRYLOCK_PI), robust futexes, vruntime tracking, min-vruntime SCHED_OTHER selection
P5: setpriority/getpriority, pthread_setaffinity_np, pthread_setname_np, pthread_setschedparam (Redox)
P6: Cache-affine scheduling (last_cpu + vruntime bonus), NUMA topology kernel hints + numad userspace daemon

Stability fixes: make_consistent stores 0 (dead TID fix), cond.rs error propagation, SPIN_COUNT adaptive spinning, Sys::open &str fix, PI futex CAS race, proc.rs lock ordering, barrier destroy

Patches: 33 kernel + 58 relibc patches, all tracked in recipes
Docs: KERNEL-SCHEDULER-MULTITHREAD-IMPROVEMENT-PLAN.md updated, SCHEDULER-REVIEW-FINAL.md created
Architecture: NUMA topology parsing stays userspace (numad daemon), kernel stores lightweight NumaTopology hints

local/patches/kernel/P8-load-balance.patch

@@ -0,0 +1,146 @@
+diff --git a/src/percpu.rs b/src/percpu.rs
+--- a/src/percpu.rs
++++ b/src/percpu.rs
+@@ -29,12 +29,14 @@ pub struct PerCpuSched {
+     pub run_queues_lock: AtomicBool,
+     pub balance: Cell<[usize; RUN_QUEUE_COUNT]>,
+     pub last_queue: Cell<usize>,
++    pub last_balance_time: Cell<u128>,
+ }
+ 
+ impl PerCpuSched {
+     pub const fn new() -> Self {
+         const EMPTY: VecDeque<WeakContextRef> = VecDeque::new();
+         Self {
+             run_queues: SyncUnsafeCell::new([EMPTY; RUN_QUEUE_COUNT]),
+             run_queues_lock: AtomicBool::new(false),
+             balance: Cell::new([0; RUN_QUEUE_COUNT]),
+             last_queue: Cell::new(0),
++            last_balance_time: Cell::new(0),
+         }
+     }
+diff --git a/src/context/switch.rs b/src/context/switch.rs
+--- a/src/context/switch.rs
++++ b/src/context/switch.rs
+@@ -33,6 +33,8 @@ const SCHED_PRIO_TO_WEIGHT: [usize; 40] = [
+     70, 56, 45, 36, 29, 23, 18, 15,
+ ];
+ 
++const LOAD_BALANCE_INTERVAL_NS: u128 = 100_000_000;
++
+ static SCHED_STEAL_COUNT: AtomicUsize = AtomicUsize::new(0);
+@@ -101,6 +103,9 @@ pub fn tick(token: &mut CleanLockToken) {
+     let new_ticks = ticks_cell.get() + 1;
+     ticks_cell.set(new_ticks);
+ 
++    let balance_time = crate::time::monotonic(token);
++    maybe_balance_queues(token, percpu, balance_time);
++
+     // Trigger a context switch after every 3 ticks.
+     if new_ticks >= 3 {
+         switch(token);
+@@ -427,6 +432,92 @@ fn steal_work(
+ 
+     None
+ }
++
++fn queue_depth(percpu: &PercpuBlock) -> usize {
++    let mut sched_lock = SchedQueuesLock::new(&percpu.sched);
++    unsafe {
++        sched_lock
++            .queues_mut()
++            .iter()
++            .map(|queue| queue.len())
++            .sum()
++    }
++}
++
++fn migrate_one_context(
++    token: &mut CleanLockToken,
++    source_id: LogicalCpuId,
++    target_id: LogicalCpuId,
++    switch_time: u128,
++) -> bool {
++    let Some(source) = get_percpu_block(source_id) else {
++        return false;
++    };
++    let Some(target) = get_percpu_block(target_id) else {
++        return false;
++    };
++
++    let source_idle = source.switch_internals.idle_context();
++    let moved = {
++        let mut source_lock = SchedQueuesLock::new(&source.sched);
++        let source_queues = unsafe { source_lock.queues_mut() };
++        pop_movable_context(token, source_queues, target_id, switch_time, &source_idle)
++    };
++
++    let Some((prio, context_ref)) = moved else {
++        return false;
++    };
++
++    let mut target_lock = SchedQueuesLock::new(&target.sched);
++    unsafe {
++        target_lock.queues_mut()[prio].push_back(context_ref);
++    }
++    true
++}
++
++fn maybe_balance_queues(token: &mut CleanLockToken, percpu: &PercpuBlock, balance_time: u128) {
++    if crate::cpu_count() <= 1 || percpu.cpu_id != LogicalCpuId::BSP {
++        return;
++    }
++    if balance_time.saturating_sub(percpu.sched.last_balance_time.get()) < LOAD_BALANCE_INTERVAL_NS
++    {
++        return;
++    }
++
++    percpu.sched.last_balance_time.set(balance_time);
++
++    let mut depths = Vec::new();
++    let mut total_depth = 0usize;
++    for raw_id in 0..crate::cpu_count() {
++        let cpu_id = LogicalCpuId::new(raw_id);
++        let Some(cpu_percpu) = get_percpu_block(cpu_id) else {
++            continue;
++        };
++        let depth = queue_depth(cpu_percpu);
++        total_depth += depth;
++        depths.push((cpu_id, depth));
++    }
++
++    if depths.len() <= 1 || total_depth == 0 {
++        return;
++    }
++
++    let avg_depth = (total_depth + depths.len().saturating_sub(1)) / depths.len();
++
++    for target_index in 0..depths.len() {
++        if depths[target_index].1 != 0 {
++            continue;
++        }
++
++        let mut source_index = None;
++        let mut source_depth = 0usize;
++        for (idx, &(_, depth)) in depths.iter().enumerate() {
++            if idx == target_index {
++                continue;
++            }
++            if depth > avg_depth + 1 && depth > source_depth {
++                source_index = Some(idx);
++                source_depth = depth;
++            }
++        }
++
++        let Some(source_index) = source_index else {
++            continue;
++        };
++
++        let source_id = depths[source_index].0;
++        let target_id = depths[target_index].0;
++        if migrate_one_context(token, source_id, target_id, balance_time) {
++            depths[source_index].1 = depths[source_index].1.saturating_sub(1);
++            depths[target_index].1 += 1;
++        }
++    }
++}