fix(python): expose FTS build memory limits (#3796)

## Summary

- expose `memory_limit` and `num_workers` on the Python FTS
configuration for local builds
- forward both build-only settings to the Lance inverted-index builder
- add an end-to-end regression proving the configured memory budget
reaches the native build

## Root cause

LanceDB 0.26.1 pinned Lance 1.0.1. That Lance version used an FTS
partition-merge path whose retained data made memory grow with merge
progress on very large indexes. Upstream Lance
[#5754](https://github.com/lance-format/lance/pull/5754) changed
partition merging to stream its inputs, reducing peak memory by about
25%. Lance [#6174](https://github.com/lance-format/lance/pull/6174) then
removed the old merge phase, compressed posting lists during
construction, reduced indexing memory by about 60%, and introduced a
total build `memory_limit` for bounded workers.

Current `main` pins Lance 11.0.0-beta.3, which contains those
architectural fixes. This PR does not duplicate or claim the upstream
leak fix; it addresses the remaining Python API gap.

## This repair

LanceDB Python did not expose the native FTS builder resource controls.
`memory_limit` now sets the total local-build budget in MiB, divided
among effective workers, and `num_workers` controls build parallelism.
Both are build-only settings and do not affect remote builds or
persisted index configuration.

## Validation

- `cargo check --quiet --features remote --tests --examples`
- `cargo fmt --all`
- `uv run --project python --extra tests --extra dev ruff check .`
- `uv run --project python --extra tests --extra dev ruff format --check
python/python/lancedb/index.py python/python/tests/test_fts.py`
- `uv run --project python --extra tests pytest python/tests/test_fts.py
-q` (51 passed)

Fixes #2923

<!-- lance-gatekeeper-fix:v1 agent=a1ceedf74531e0212cb6f1ebf9390a26
generation=1 -->

---------

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

python/Cargo.toml

@@ -26,7 +26,9 @@ lance-namespace-impls.workspace = true
 lance-io.workspace = true
 env_logger.workspace = true
 log.workspace = true
-pyo3 = { version = "0.28", features = ["extension-module", "abi3-py310", "chrono"] }
+# Maturin enables extension-module mode for Python builds. Keeping it out of
+# Cargo features lets Rust unit tests link against libpython.
+pyo3 = { version = "0.28", features = ["abi3-py310", "chrono"] }
 chrono.workspace = true
 pyo3-async-runtimes = { version = "0.28", features = [
     "attributes",
@@ -41,10 +43,7 @@ tokio.workspace = true
 libc = "0.2"
 
 [build-dependencies]
-pyo3-build-config = { version = "0.28", features = [
-    "extension-module",
-    "abi3-py310",
-] }
+pyo3-build-config = { version = "0.28", features = ["abi3-py310"] }
 
 [features]
 default = ["remote", "lancedb/aws", "lancedb/gcs", "lancedb/azure", "lancedb/dynamodb", "lancedb/oss", "lancedb/huggingface", "lancedb/cos", "lancedb/goosefs", "lancedb/metrics-otel"]