fix(cbq): read_index / MmapReader crash on zero-record CBQ files

Summary

Index::from_bytes fails on empty input, so a valid CBQ file containing
zero records (e.g. a run where an upstream filter discarded every read) cannot
have its index read: both the streaming Reader::read_index and
MmapReader::new return CbqError::IndexCastingError
("Unable to cast bytes to Index - likely an alignment error"). This makes
bqtools decode (and any other index-reading consumer) crash on such files.

Root cause

A zero-record CBQ has an empty index: u_bytes = 0 and a 9-byte zstd frame of
zero-length content. After decompression the buffer is an empty Vec<u8>,
whose pointer is dangling and aligned only to 1.
bytemuck::try_cast_slice::<u8, BlockRange> checks pointer alignment even for
empty slices, so it returns TargetAlignmentGreaterAndInputNotAligned.

The bytes on disk are canonical — they are exactly what ColumnarBlockWriter
produces for empty input — so this is purely a reader-side issue.

Fix

Guard the empty case before casting (src/cbq/core/index.rs):

if bytes.is_empty() {
    return Ok(Self { ranges: Vec::default() });
}

Test

Added cbq::read::tests::test_read_index_empty_file
(src/cbq/read.rs): writes a zero-record CBQ via ColumnarBlockWriter,
reads it back with the streaming Reader, and asserts the index loads with
0 records / 0 blocks.

• Fails on current code with CbqError(IndexCastingError) — passes with the fix.
• Full suite green: cargo test (327 tests + doctests), cargo fmt --check, cargo clippy.