fix: restore simple FTS tokenizer default

[Xuanwo]

This restores the native FTS default tokenizer to simple after ICU showed behavior differences that are too large for the default path. ICU remains available through explicit base_tokenizer="icu", while docs and tests now describe the default as simple again.

[github-actions]

Important

This PR touches the Lance format specification.

Substantive changes to the format specification — the .proto definitions
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[Xuanwo]

This PR touches the Lance format specification.

Not really

[wjones127]

+1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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