Speed up by reusing tokio instance #6

Claude claims this is faster

  Changes:
  1. Added rt: tokio::runtime::Runtime field to PdsFs<R> struct
  2. Initialize runtime once in new() method
  3. Replaced two instances of Runtime::new().unwrap():
    - In attr() method (line 169-171) → now uses self.rt
    - In read() method (line 388-391) → now uses self.rt

  Measured performance improvement:
  - Before: 10.981 seconds (cold cache)
  - After: 1.769 seconds (cold cache)
  - Speedup: 6.2x faster

  The optimization eliminates the massive overhead of creating ~22,000 Tokio runtimes (one per file) during directory
   listing operations. Builds cleanly with no warnings.