feat: weight allocations by apy and headroom

[dangelo352]

Summary

• weight allocation within eligible risk buckets by apy * available_headroom with checked arithmetic
• fall back to headroom-only weights when every eligible pool has zero APY
• assign integer-division dust by largest fractional remainder, then spill capped shortfalls in registry order
• preserve strategy bucket splits and capacity/inactive-pool invariants
• add formula docs and benchmark coverage

Fixes #473

Tests

• CARGO_TARGET_DIR=/tmp/commitlabs-weighted-allocation-target cargo test -p allocation_logic weighted -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-weighted-allocation-target cargo test -p allocation_logic allocate -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-weighted-allocation-target cargo test -p allocation_logic rebalance -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-weighted-allocation-bench-target cargo test -p allocation_logic --features benchmark benchmark_allocate -- --nocapture