ENH: Add initial BCI2000 .dat reader (preload-only)

[HansujaB]

Reference issue (if any)

Fixes #13151

What does this implement/fix?

This PR adds basic support for reading BCI2000 .dat files via:

mne.io.read_raw_bci2k()

The implementation:

• Parses minimal required header fields
• Decodes signal and state vector
• Maps StimulusCode to STI 014 stim channel
• Returns a Raw object using RawArray
• Includes deterministic unit tests with a synthetic demo file

Tests

• Synthetic .dat generator included in tests

• Verifies:

  • sampling rate
  • number of samples
  • stim channel creation
  • correct StimulusCode decoding
  • preload=False raises

Scope (Intentional Limitations)

• preload=False is not supported (raises NotImplementedError)
• Minimal header parsing (SamplingRate + State Vector Definition only)
• Generic channel names (EEG1, EEG2, …)
• No gain/scaling applied
• Not added to _get_supported() (since .dat is already mapped to Curry)

The goal is to provide a correct, minimal reader that can be safely extended in follow-up PRs.