fix(fifo): implement proper blocking for FIFO read/write operations

[ryanbreen]

Summary

• Implement proper blocking for FIFO read operations with HLT loop
• Fix critical lock contention bug that prevented context switches during blocking
• Add read_waiters tracking to PipeBuffer for wake-on-data notifications
• Wake blocked readers when data is written or all writers close (EOF)

Problem

Phase 7 of the FIFO tests (blocking read with fork) was failing because:

1. Parent process blocked on FIFO read while holding process manager lock
2. Timer interrupts tried to context switch to child process
3. Context switch failed: "Could not acquire process manager lock to restore context"
4. Child never executed, never wrote data, parent blocked forever

Solution

Drop the process manager lock before entering blocking paths:

FdKind::FifoRead(_path, pipe_buffer) => {
    let pipe_buffer_clone = pipe_buffer.clone();
    drop(manager_guard);  // CRITICAL: Release lock before blocking!
    // ... blocking loop ...
}

The blocking implementation follows the established TCP double-check pattern:

1. Register as waiter BEFORE setting blocked state (race fix)
2. Block thread and set blocked_in_syscall flag
3. Double-check for data arrival (race condition fix)
4. HLT loop until woken
5. Retry read after wake

Test Results

All 10 FIFO test phases now pass:

• Phase 1-6: Basic FIFO operations
• Phase 7: Blocking read with fork (was failing)
• Phase 8: EOF test (reader gets 0 when writer closes)
• Phase 9: EPIPE test (write fails when reader closes)
• Phase 10: Unlink FIFO while open

Files Changed

• kernel/src/ipc/pipe.rs - Add read_waiters tracking and wake mechanism
• kernel/src/ipc/fifo.rs - Add without_interrupts wrappers for lock safety
• kernel/src/syscall/fs.rs - FIFO open blocking implementation
• kernel/src/syscall/handlers.rs - FIFO read/write blocking with proper lock release

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