PERF: speed up tab completion for DataFrame/Series with a large index

[jbrockmendel]

I'm on the fence as to whether this is a big enough deal to merit special-casing.

Summary

• closes PERF: tab completion with a large index #18587
• Avoid hashing the entire Index inside _dir_additions_for_owner when only the first display.max_dir_items unique values are needed. First try a bounded prefix (max_items * 10); fall back to full unique() only when the prefix is duplicate-heavy, so behavior is preserved for pathological sorted-duplicate cases (e.g. MultiIndex level 0).
• First dir(df) for a DataFrame with 1M unique string columns goes from ~115 ms to ~17 ms on my machine. Non-unique and sorted-duplicate cases are unchanged.
• Adds an ASV Dir benchmark (cold path via number=1 + _cache={}) and a whatsnew entry.

Test plan

• pytest pandas/tests/frame/test_api.py pandas/tests/series/test_api.py pandas/tests/indexes/test_base.py
• test_display_max_dir_items covers the default cap, a custom cap, and None (unlimited)
• pre-commit run on the changed files

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