Support u128 bitpacking for decimal128 columns

Background

The miniblock encoder picks an inline bitpacking width by walking {8, 16, 32, 64} and choosing the narrowest one that fits the column's value range, but anything wider — i.e. decimal128 — falls through to raw 128-bit storage today, even when the actual values comfortably fit in 32 or 64 bits. In practice, real-world decimal128 columns almost never use the full 128-bit range; money, prices, taxes, accounting figures all sit in narrow ranges, so a non-trivial fraction of every dataset that uses decimal128 is being stored at the wrong bit-width.

Impact

I measured this on TPC-DS SF=100 store_sales (288 M rows, 23 columns including 12 × decimal128(7,2)):

A ~53 % reduction entirely on the decimal128 columns, with schema, row count, and file format version (v2.1) all unchanged. This isn't a TPC-DS quirk — decimal(7,2) only needs ~24 bits of actual range, and that pattern repeats across most real decimal columns I've seen.

Proposal

Extend the miniblock bitpacking chooser to also consider bits = 128 and route that case through a scalar BitPacking kernel for u128. The picked width is still chosen the same way (narrowest that fits), so a pathological column that genuinely uses the full 128-bit range stays at 128 bits and pays nothing extra.

Scope of the change is narrow:

• rust/compression/bitpacking/ — add the scalar u128 kernel.
• rust/lance-encoding/src/encodings/physical/bitpacking.rs — wire u128 through the miniblock encode/decode path.
• rust/lance-encoding/src/compression.rs — extend the chooser to match bits ∈ {8, 16, 32, 64, 128}.
• rust/lance-encoding/src/statistics.rs — minor stat-related plumbing.

No new public API, no on-wire format change beyond a bit-width that v2.1 readers already accept (the encoder just didn't previously emit it), and no FastLanes-transposed kernel for u128 — scalar only for now, that's a natural follow-up.

Limitations

• Decimal128-only. Other types are unchanged.
• Range-dependent. Columns whose values genuinely span the full 128-bit range won't compress and fall through to raw 128 just like today.

Start in rust/compression/bitpacking/ for the scalar u128 kernel, then trace the miniblock encode/decode path in rust/lance-encoding/src/encodings/physical/bitpacking.rs. Check the chooser in rust/lance-encoding/src/compression.rs and the related plumbing in rust/lance-encoding/src/statistics.rs. Done means decimal128 values use the narrowest supported width through 128 bits and round-trip correctly without changing other types.