Add file-backed ring buffer with per-topic configurability

[Copilot]

The in-memory ring buffer can consume excessive RAM for high-bandwidth topics like camera images. This adds a file-backed alternative that keeps only a small index in RAM while writing payloads to a temporary file, selectable per topic.

New: FileRingBuffer

• Drop-in replacement for RingBuffer (identical API: add, snapshot, size, memory_bytes)
• Payloads stored in tempfile.TemporaryFile (auto-deleted on GC/close); in-memory index holds only (timestamp_ns, file_offset, record_size) per message
• Automatic file compaction when dead bytes (purged records) exceed live bytes

Config: per-topic buffer key

topics:
  - type: "sensor_msgs/msg/Image"
    buffer: file        # new — file-backed; default is "memory"
    max_frequency: 1.0
    qos: { ... }

  - pattern: "/**"
    buffer: memory      # explicit default; in-memory RingBuffer (unchanged behaviour)

example_config.yaml updated to use buffer: file for sensor_msgs/msg/Image.

Node changes (ringbuffer_node.py)

• Topics with buffer: file get a dedicated FileRingBuffer(duration) stored in self._file_bufs
• Snapshot merges messages from the shared memory buffer and all per-topic file buffers, sorted by timestamp before writing the rosbag

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