deps: bump the iceberg-rust pin to bb1e4a4 and document why it is pinned

[andygrove]

Which issue does this PR close?

Part of the native Iceberg writes epic, #5649.

This is the kind of periodic bump #5645 asks for, but it does not close it: #5645 wants the
policy written down, and this PR only performs one bump and documents why the dependency is a
git revision at all. Leaving #5645 open.

Rationale for this change

native/Cargo.toml pins iceberg and iceberg-storage-opendal to 665c64e4 (2026-09-03).
Upstream is now 51 commits ahead, and the pin has no note explaining itself, so the reasonable
question "can we just use a release?" has to be re-derived from scratch each time someone looks.

The answer is no, not yet: the latest published iceberg-rust is 0.10.1 (2026-08-01), which
requires arrow/parquet ^58, while Comet is on 59.2. Those ranges are disjoint, and since the
iceberg-rust writer API takes arrow_array::RecordBatch directly, a v58 RecordBatch is a
different type from the v59 one we hand it — a compile error, not just a duplicate crate. The
currently pinned revision is in fact upstream's "deps: upgrade to DataFusion 55.0 and
Arrow/Parquet 59.2" commit, i.e. the pin has always been an arrow-alignment pin rather than a
missing-API one. Every iceberg:: path the native crate references already exists in 0.10.1.

So the pin stays, but two things are worth doing now:

1. Move it forward. Several of the 51 commits land in the write path the native writer
   actually drives: perf(arrow): move partition key into group map instead of cloning per row iceberg-rust#3159 removes a per-row partition-key clone in the record
   batch partition splitter, perf(arrow): build partition filter mask with BooleanBufferBuilder iceberg-rust#3204 and perf(arrow): build constant filter masks with BooleanBuffer instead of Vec<bool> iceberg-rust#3177 replace
   Vec<bool> masks with BooleanBuffer, and perf(writer): borrow statistics in MinMaxColAggregator::update instead of cloning iceberg-rust#3171 stops cloning statistics
   in MinMaxColAggregator::update (runs per column chunk). These are upstream's own performance
   claims; this PR does not measure them in Comet's context.
2. Write down why the pin exists, matching the comment convention already used for the
   regex pin a few lines above, so the next reader gets the arrow constraint and the exit
   condition without redoing the analysis.

Two things that look like reasons to bump but are not, checked and recorded so they are not
re-litigated: the rustls 0.23.45 fix (apache/iceberg-rust#3244) does not reach us — our lock
resolves rustls to 0.23.44 on both revisions — and the arrow-parquet group bump
(apache/iceberg-rust#3209) is a no-op since we are already on parquet 59.3.0 either way. This
bump also does not unblock #5898; apache/iceberg-rust#3111 is still open.

What changes are included in this PR?

• native/Cargo.toml / native/Cargo.lock: move both iceberg crates from 665c64e4 to
  bb1e4a48 (2026-09-21). The lockfile change is confined to the three iceberg entries — the
  resolved graph is otherwise identical, same 615 crates with no version changes on either side,
  so there is no transitive churn to review.
• A three-line comment above the pin recording the arrow constraint, the exit condition (a
  release on arrow 59, at which point this becomes a plain version requirement), and a pointer
  to Adopt a pin-bump policy for the iceberg-rust dependency #5645.

No Comet code changes were needed, despite heavy upstream churn in modules we import
(scan/mod.rs +442/-30, scan/task.rs +221/-50, arrow/value.rs, spec/partition.rs,
record_batch_partition_splitter.rs).

How are these changes tested?

By the existing suites; there is no new behavior to test.

Locally on the final state of the branch:

• cargo check --workspace --all-targets — clean, no errors or warnings.
• cargo clippy --workspace --all-targets -- -D warnings — clean.
• cargo fmt --all -- --check — clean.
• cargo test -p datafusion-comet iceberg — 74 passed, 0 failed. This includes the parity tests
  that would catch an upstream behavior change in the writer:
  iceberg_rust_transform_parity::{bucket,truncate,years_and_months}_agrees_with_iceberg_rust,
  partition_spec_renders_like_iceberg_java, the *_renders_like_iceberg_java partition-path
  cases, encoded_manifest_round_trips_through_iceberg_parser, the rolling-grid cases, and
  test_metadata_field_id_constants_match_iceberg_rust.
• cargo test -p datafusion-comet --lib — 430 passed, 0 failed.

The Rust tests are not sufficient sign-off on their own here: the native scan's real coverage is
the JVM Iceberg suites, and scan/mod.rs and scan/task.rs changed substantially upstream. I
have applied run-iceberg-tests so CI runs the Iceberg Spark SQL tests against every Iceberg
version, per the bump policy sketched in #5645.

[comphead]

Thanks @andygrove