fix: inherit Lance defaults for IVF RQ quantization (#4585)

`IvfRq()` forced `num_bits=1` through Python's Rust binding, overriding
Lance's five-bit default. The shared native index creation path also
substituted one bit when the Rust or Node builder left the value unset.

Make Python's bit count optional and forward only explicit values. Use
`RQBuildParams::default()` for an unset native bit count, and omit unset
values when dispatching to pylance for accelerator builds. Update the
Python, Rust, Node, and vector-index documentation.

New indexes with an unset bit count now inherit Lance's default,
currently five bits, across Python, Rust, and Node. Five-bit vector
codes use roughly five times the storage of one-bit codes, plus other
index overhead. This also changes reader compatibility: indexes created
with `num_bits >= 2` use a newer on-disk layout that some older LanceDB
versions cannot read. Upgrade readers to versions that support multi-bit
IVF_RQ, or explicitly set `num_bits=1` (for example, `IvfRq(num_bits=1)`
in Python) when sharing indexes with readers limited to one-bit IVF_RQ.

Validation:

- Reproduced six regression failures before the fix: one-bit default
metadata, rejected `None`, and incorrect remote/accelerator forwarding.
- After merging `main` through `b762ec22`, rebuilt both native bindings
against Lance 15.0.0-beta.1. Python validation passed with 106 tests and
15 optional Ollama skips. Coverage includes the full index suite,
omitted and explicit bit counts, reading Lance-created IVF_RQ indexes,
accelerator forwarding, CUDA-unavailability handling, timeout
validation, remote requests, and table/namespace listing behavior.
- Rust formatting and Python package Ruff lint/format checks pass. Node
`pnpm lint`, `pnpm build` (including TypeScript checks), `pnpm run
docs`, the full connection suite, and the existing IVF_RQ regression
pass. Generated documentation adds no further changes.

The merge preserves upstream's explicit RaBitQ heading anchor alongside
the `IvfRq()` example and five-bit default/reader-compatibility
guidance.

Repository-wide `ruff check .` still reports 49 existing errors in
unchanged documentation examples under `docs/web-tests`; root formatting
also finds 11 unchanged documentation example files. The Python package
checks pass.

Fixes #4582

<!-- lance-gatekeeper-fix:v1 agent=6ef9406f0142b1ab66d53d9d56098223
generation=1 -->

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Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>
Co-authored-by: Xuanwo <github@xuanwo.io>

docs/src/js/interfaces/IvfRqOptions.md

@@ -58,7 +58,8 @@ Number of bits per dimension for residual quantization.
 
 This value controls how much each residual component is compressed. The more
 bits, the more accurate the index will be but the slower search. Typical values
-are small integers; the default is 1 bit per dimension.
+are small integers. If unset, uses Lance's default (currently 5 bits per
+dimension). Set to 1 for maximum compression.
 
 ***