[iaojnh]

Opening an existing buffer-backed index with create_new=false previously opened its pool as read-only, even when read_only=false. Add IndexStorage::OpenOptions and pass creation and access permissions separately from Index::open(), so an existing index can remain writable without permitting file creation.

The legacy boolean entry point keeps its existing behavior. The explicit entry point is opt-in for storage implementations, rejects read-only creation requests, and preserves the read-only bypass-only fallback. Writable pools still require cache initialization. Storage-open errors now reach the caller unchanged, including OpenFile and NoMemory.

Related to the storage-open review discussion in #833. Once this lands, #833 can use the explicit options entry point in place of its boolean OR workaround.

Validation on macOS arm64 (Release, C++17, warnings treated as errors; optional RaBitQ implementation disabled):

• buffer_storage_write_test: 60 tests passed, including writable reopen persistence, missing-file creation permissions, legacy access modes, read-only bypass fallback, and writable memory-pressure retry.
• index_interface_test: 7 selected tests passed, covering buffer-backed reopen persistence and error propagation, dirty-state tracking, IVF read-only loading, and DiskANN reads across cache budgets.
• hnsw_turbo_index_test: 18 selected reopen tests passed, including the legacy BufferStorage loading path.
• clang-format 18.1.8 and git diff --check passed.