feat: add lance_dataset_drop_columns for metadata-only column removal

[LuciferYang]

Summary

First of three PRs against #41 (schema evolution). Exposes upstream's drop_columns — a metadata-only manifest commit that removes the named columns from the schema without rewriting any data files. Materializing the projection is left to a later _compact_files (and a future cleanup operation, once exposed, removes the old version's files).

Mutates the dataset in place under an exclusive write lock; scanners already in flight keep their pre-drop snapshot view via the existing Arc clone-on-write, same as _delete / _update / _compact_files.

Surface

int32_t lance_dataset_drop_columns(
    LanceDataset* dataset,
    const char* const* columns,
    size_t num_columns
);

Inputs are validated up front with per-index error messages so the precise cause is observable from lance_last_error_message(). NULL handle, NULL pointer array, zero count, NULL or empty-string entries, and non-UTF-8 names all return LANCE_ERR_INVALID_ARGUMENT; upstream's own rejections (unknown column, attempt to drop every column) map to the same code.

The C++ wrapper takes const std::vector<std::string>& and follows the update / merge_insert sibling convention — passes col_ptrs.data() unconditionally. An empty vector flows through the Rust-side num_columns == 0 guard so the error message says "num_columns must be > 0" rather than the misleading "columns must not be NULL".

Tests

Eleven new Rust integration tests covering single-drop, multi-drop, version bump, data preservation (downcasts the surviving Arrow columns and checks the actual values, not just shape), and the full rejection surface (NULL dataset / NULL array / zero count / NULL entry / empty-string entry / unknown column / drop-all). C and C++ smoke tests snapshot ArrowSchema.n_children pre/post drop, exercise the drop-last-column rejection path, and verify the version is unchanged when a drop fails. cargo test and cargo test --test compile_and_run_test -- --ignored both green.

Follow-ups

• lance_dataset_alter_columns — rename / nullability / type change
• lance_dataset_add_columns — SQL expressions / AllNulls / ArrowArrayStream

The README roadmap entry stays unticked until all three ship.

[LuciferYang]

The macOS arm64 leg of consumer-smoke-test failure here is pre-existing on main (failing since #24 — same unresolved _IO* symbols from sysinfo / objc2_io_kit), not introduced by this PR. Sent #43 as a focused fix to declare -framework IOKit in the CMake / pkg-config link line; once that merges, a rerun here should go green.