feat: add batch allocation entrypoint

[dangelo352]

Summary

• add BatchAllocateParams/BatchAllocateResult plus admin-gated batch_allocate using shared BatchProcessor limits and BatchMode semantics
• keep single allocate behavior shared through an internal allocation helper, with distinct caller auth handling for batch items
• preflight Atomic mode against simulated pool liquidity so failures do not write partial allocations
• add BestEffort partial-failure reporting, capacity-failure coverage, empty/max-batch tests, and benchmark entrypoint coverage
• document batch allocation semantics and limits

Fixes #476

Tests

• CARGO_TARGET_DIR=/tmp/commitlabs-batch-allocation-target cargo test -p allocation_logic batch_allocate -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-batch-allocation-target cargo test -p allocation_logic allocate -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-batch-allocation-target cargo test -p allocation_logic rebalance -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-batch-allocation-bench-target cargo test -p allocation_logic --features benchmark benchmark_batch_allocate -- --nocapture