Metric: liquidity concentration

Written to the standard set by #33 — read that issue first for the expected level of detail and the acceptance criteria every check shares. ## What Measure how concentrated a market's liquidity is — how much of the available depth comes from how few offers or accounts. ## Why it matters Two books with identical total depth are not equally safe. One with a hundred offers from thirty accounts survives any single participant leaving. One where 90% of depth is a single offer from one account is one cancellation away from being the KESC corridor. Total depth alone cannot distinguish them, and the difference is the whole risk. ## Which layer **Layer 2 — deterministic calculation.** ## This is a metric `MetricResult`, `UnitRatio`. **No threshold** — see [docs/checks.md](https://github.com/Wayfare-labs/wayfare/blob/main/docs/checks.md). ## Exactly where the code goes - **New file:** `checks/metric_concentration.go` - Tests appended to `checks/checks_test.go` - Reads `dex.Client.OrderBook` ## Where the data comes from Horizon `/order_book`. Note the caveat already documented in `dex/health.go`: the meaning of the `amount` field is genuinely ambiguous between base and counter asset, and `BookHealth` deliberately avoids depending on it. **Read that comment before deciding what to measure** — a concentration figure built on a misread amount would be confidently wrong. Consider measuring concentration over **offer count and price levels** rather than amounts, or state clearly which interpretation you took and why. ## What "unmeasurable" means here - Empty or one-sided book → `Determined: false`, not zero - A single offer → concentration is definitionally total; report it as determined with the count, not as an error - Horizon does not expose the offering account in `/order_book` — if account-level concentration is wanted it needs `/offers`, which is a **separate issue**. Say plainly which you measured ## Acceptance criteria - [ ] `MetricResult` with `UnitRatio` - [ ] `decimal.Decimal` throughout; no float64 - [ ] Returns UNABLE-TO-DETERMINE (not zero) for an empty or one-sided book - [ ] States in the descriptor's `CannotDetermine` whether account-level concentration was measured, and if not, why - [ ] Does not depend on the ambiguous `amount` field without saying so explicitly - [ ] Evidence names the levels observed - [ ] Table tests from `dex/testdata/orderbook-xlm-ngnc.json` - [ ] Negative test: empty book, and a single-offer book - [ ] No new third-party dependencies - [ ] gofmt, vet, test -race, golangci-lint clean ## Out of scope Nothing in `dex/`. No `/offers` crawling — that is a separate issue if wanted. ## How to verify locally ```bash make offline-test go test ./checks/ -run TestConcentration -v ```