refactor: decompose price calculation into orthogonal components

[suchapalaver]

Summary

PriceCalculator is now a thin orchestration façade over five focused submodules instead of one type that mixed scanning, decoding, decimals fetching/caching, normalization, and aggregation. This makes the price domain testable without a provider and easier to extend for alternate metadata or aggregation strategies. PriceSource stays the DEX extension point and the public API is unchanged. Closes #26.

What's in the diff

• src/price/extractor.rs (new) — pure extract_swaps that walks logs through PriceSource::extract_swap_from_log + should_include_swap, tracing decode/invalid-swap errors without aborting.
• src/price/normalize.rs (new) — pure normalize_against_pair (target/USDC, both directions), normalize_swap, and the involves_pair relevance predicate.
• src/price/aggregator.rs (new) — PriceAggregator folds normalized swaps into a TokenPriceResult.
• src/price/decimals.rs (new) — provider-free TokenDecimalsCache plus TokenMetadataProvider (single + parallel batch fetch, CallBatchLayer-friendly).
• src/price/scanner.rs (new) — SwapLogScanner builds the router/topics filter and drives the chunked scan.
• src/price/calculator.rs — rewritten as orchestration; keeps the public PriceCalculator, TokenPriceResult, RawSwapResult and their methods. Review first: process_gap_for_price and extract_raw_swaps.
• src/price/mod.rs — declares the new private submodules.

Acceptance check (from #26)

• Price aggregation testable with in-memory SwapData, no provider — aggregator.rs + normalize.rs unit tests.
• Token decimal cache testable independently — decimals.rs TokenDecimalsCache tests.
• PriceCalculator is smaller and reads as orchestration.
• Existing price tests pass (range-cache + TokenPriceResult tests retained).
• examples/custom_dex_integration.rs still compiles — cargo check --examples --all-features.
• PriceSource remains the documented extension point.
• new, with_config, calculate_price_between_blocks, raw-swap API kept source-compatible.
• Batch decimals fetch + CallBatchLayer compatibility preserved (pair-scoped batch retained).

Test plan

• cargo test --all-features — 509 lib + all integration suites pass
• cargo test / cargo test --no-default-features — pass
• cargo clippy --all-targets --all-features -- -D warnings clean (all three feature combos)
• cargo fmt --check clean
• cargo check --examples --all-features clean

Self-review notes

Pre-commit /code-review (5 angles, extra-high effort) found the refactor behavior-faithful. One pre-existing operator-facing gap surfaced — a persistent decimals-fetch failure caches an empty/partial TokenPriceResult for the range, so a later same-range query returns the bogus zero without rescanning. This is not introduced here (the original process_gap_for_price cached skipped-swap results identically), so it's left out of scope and filed as a follow-up.

[suchapalaver]

Self-review pass

Pre-commit /code-review (5 angles, extra-high effort) + post-PR /pr-review (independent structural audit), findings triaged below.

#	Finding	Status
1	Two price tests asserted on raw U256→f64 arithmetic — testing a dependency, not crate code (CLAUDE.md violation)	Fixed: baf2c45 (removed; normalize module now covers this)
2	Persistent decimals-fetch failure caches an empty/partial result for the range, defeating retry on a later same-range query	Filed: #66 — pre-existing (original process_gap_for_price cached skipped-swap results identically), out of scope for a behavior-preserving refactor
3	TokenMetadataProvider fetch/skip/retry path is untested (only the TokenDecimalsCache wrapper is)	Skipped: the acceptance criterion ("token decimal cache testable independently") is met by the cache tests; the fetcher is a thin wrapper over LazyToken::decimals() and the crate keeps RPC-dependent paths in examples/ with no mock provider
4	PriceAggregator is a thin wrapper; TokenPriceResult could live in aggregator.rs	Skipped (nit): TokenPriceResult is a public re-exported type; keeping it in calculator.rs avoids churn, and the named fold reads clearly in orchestration

Both reviews independently concluded the decomposition is complete and behavior-preserving (no dead code, no leaked internal pub, public API and mod.rs workflow rustdoc still accurate). No blockers remaining — confident to ship.