Support variable fee per UniswapV2 pool

[Haikane]

Summary

• Add fee_bps field to UniswapV2State (defaults to 30, serde-compatible with existing serialized states)
• Decoder reads fee from static_attributes on the component snapshot
• All fee calculations use per-pool fee instead of hardcoded constant
• Bounds validation: new_with_fee() asserts fee_bps <= 10000 to prevent arithmetic underflow in fee_numerator(). Decoder returns ValueError for out-of-range fees.

Test plan

• 28/28 UniV2 unit tests pass (state + decoder)
• 38/38 query_pool_swap tests pass
• Custom fee decoding test (fee=25)
• Edge case tests: fee=0, fee=10000, fee>10000 (rejected), missing fee (defaults to 30), empty fee bytes
• Full crate compiles with all features

🤖 Generated with Claude Code

[Haikane]

Review fixes pushed

Addressed findings from the multi-agent code review:

Bounds validation (d12e2af1)

• state.rs:new_with_fee() — Added assert!(fee_bps <= 10000) to prevent arithmetic underflow in fee_numerator() where 10000 - fee_bps would wrap for values > 10000.
• decoder.rs — Added InvalidSnapshotError::ValueError check when decoding fee from static_attributes, so invalid fee values from the indexer are caught at decode time instead of panicking later.

Edge case tests (d12e2af1)

Added 7 new tests:

• test_new_with_fee_exceeds_max — verifies panic on fee > 10000
• test_new_with_fee_zero — verifies zero-fee pool (numerator = 10000)
• test_new_with_fee_boundary — verifies 100% fee edge (numerator = 0)
• test_usv2_try_from_missing_fee_defaults_to_30 — verifies default path
• test_usv2_try_from_zero_fee — verifies zero-fee decode
• test_usv2_try_from_fee_exceeds_max — verifies decoder rejects > 10000
• test_usv2_try_from_empty_fee_bytes — verifies empty bytes => fee=0

All 28 uniswap_v2 tests pass.