Fix stale buffer detection in Openable.getBuffer() for recreated jar files

[Copilot]

PR #14 placed stale buffer detection in ClassFile.openBuffer() and ModularClassFile.openBuffer(), but openBuffer() is only called when the cache returns null. When a jar is deleted and recreated with the same path, the BufferManager cache returns the old (closed) buffer — openBuffer() is never reached.

Changes

• Openable.getBuffer() — Essential fix. Detect closed buffers from cache before they're returned:

  IBuffer buffer = getBufferManager().getBuffer(this);
  if (buffer != null && !(buffer instanceof NullBuffer) && buffer.isClosed()) {
      getBufferManager().removeBuffer(buffer);
      buffer = null;
  }

• ClassFile.openBuffer() — Defense-in-depth. Same isClosed() check for the outermost enclosing type's cached buffer, covering inner class lookups.
• ModularClassFile.openBuffer() — Left unmodified. PR Fix stale buffer reuse when jar files are recreated with same path #14 incorrectly added a cache lookup that didn't previously exist and isn't needed.
• ClassFileTests — Added testStaleBufferAfterJarRecreation() covering the create→delete→recreate jar cycle.

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