Add opt-in RLE v2 run-length widths

[Xuanwo]

Summary

Add an opt-in RLE v2 codec path that supports run_lengths = Flat(8|16|32) without changing the Lance data file format or RLE protobuf shape.

Scope

• Decode RLE run lengths from Flat(8), Flat(16), and Flat(32).
• Keep Flat(8) as the default compatibility mode.
• Add strict validation for zero lengths, values/lengths entry mismatches, underflow, and overflow.
• Add a table-level reader/writer feature flag for datasets that may contain RLE v2 pages.
• Add an explicit writer opt-in path.

Acceptance Criteria

• New Lance can write and read opt-in RLE v2 pages.
• Old Flat(8) RLE data remains readable.
• Old readers fail closed at the table level when RLE v2 pages may exist.
• Compatibility mode still writes Flat(8).