Add --batch NR_BITS parameter to bench_compare binary to benchmark
get_batch() and put_batch() APIs:
- When --batch > 1, runs batch mode benchmarking only Sbitmap
- When --batch = 1 (default), runs normal mode with both implementations
- Validates batch_size <= bits_per_word() constraint
- Refactor benchmark code to share common logic via benchmark_internal()

This allows measuring performance of consecutive bit allocation/deallocation
under high concurrency scenarios.

Signed-off-by: Ming Lei <[email protected]>

Add init_hint() helper to eliminate code duplication between
run_workload() and run_batch_workload(). Both functions now use
the same hint initialization logic that combines stack address
and system time for pseudo-random distribution.

Signed-off-by: Ming Lei <[email protected]>

Update README.md to document the new batch allocation APIs:
- Add "Batch Allocation" usage example showing how to allocate
  and free consecutive bits atomically
- Document get_batch() and put_batch() methods in API section
- Add batch operations to Memory Ordering guarantees
- Update Performance Characteristics with batch complexity
- Add batch resource allocation to Use Cases
- Document --batch parameter in benchmarks section

Signed-off-by: Ming Lei <[email protected]>

Signed-off-by: Ming Lei <[email protected]>

Split test_batch_word_boundary into two versions:
- test_batch_word_boundary_64bit for 64-bit systems (shift=6, 64 bits/word)
- test_batch_word_boundary_32bit for 32-bit systems (shift=5, 32 bits/word)

This fixes the shift overflow error on i686 where usize is 32 bits
and cannot support 64-bit words.

Also fix clippy warning by using div_ceil() and inline format args.

Signed-off-by: Ming Lei <[email protected]>

When nr_bits equals BITS_PER_WORD (32 on i686), the expression
(1usize << nr_bits) causes shift overflow. This helper checks for
this special case and returns usize::MAX instead of shifting.

Applied to three locations:
- find_next_zero_batch()
- get_batch_from_word()
- put_batch()

Fixes test_batch_concurrent failure on i686 architecture.

Signed-off-by: Ming Lei <[email protected]>