refactor(arch): publish DiscountCeiling in Commerce's contract

[mforce]

Part of #1116 (cleanup PR 1). It does not close the issue.

Moves DiscountCeiling from Domain/Modules/Commerce/Sales/ to Domain/Modules/Commerce/Contracts/, unchanged apart from its namespace. Farm stops reaching a Commerce type outside the contract.

CW1004 census (tools/architecture/cw1004-census.sh): 57 → 53. The 4 removed rows are Farm → Commerce.Sales.DiscountCeiling at Account.cs:90, FarmModule.cs:64, UpdateFarmSettingsHandler.cs:150 and UpdateFarmSettingsValidator.cs:98. Three ConfirmSaleHandler rows only moved down one line because of the added using. No other row changed.

Proofs and the policy review follow in an edit.

Policy review: why DiscountCeiling belongs in Commerce's public API

Moving a type into Contracts/ changes what the guards let peers and adapters name, so this is a policy change, not just a move.

• It is already a cross-module value.

  • Farm owns the setting: Account.MaxDiscountBasisPoints is exposed as Account.MaxDiscount.
  • Farm validates and parses it: UpdateFarmSettingsValidator and UpdateFarmSettingsHandler use DiscountCeiling.TryParsePercent.
  • Farm reports it: FarmModule fills FarmSettingsDetails from .Percent.
  • Commerce applies it at confirm (ConfirmSaleHandler, SalesOrder).

  Publishing one type keeps one rule for parsing, range (MaxBasisPoints) and comparison. Otherwise Farm would need a parser of its own that could drift.

• It is a value, not behaviour or state: a readonly record struct holding basis points, with parse, format and compare members. It names no entity, aggregate or persistence type. ModuleContractRealAssemblyTests now walks it (see proofs).

• What it opens: any peer or adapter may now name DiscountCeiling. That is intended: the ceiling is part of the farm-settings surface that IFarmModule already exposes as a percent. It gives no access to orders, lines or the confirm path.

• Not done: moving ownership of the type to Farm. That would be a design change, and Commerce would then reach into Farm's contract for its own pricing rule.

Proofs (head 38cefb15)

Check	Result
Census	57 → 53. Exactly the 4 Farm → DiscountCeiling rows removed; 3 ConfirmSaleHandler rows moved down one line (added using)
Tenant-bypass re-pin	none. No member names DiscountCeiling by qualified name; TenantBypassRealTreeTests 4/4 with the registries untouched
EF model	dotnet ef migrations has-pending-model-changes: "No changes have been made to the model since the last migration" (AccountConfiguration ignores MaxDiscount; the snapshot never named the type)
Purity guard covers the promoted type	adding SalesOrder? PurityProbe to DiscountCeiling makes ModuleContractRealAssemblyTests fail ("DiscountCeiling.FromBasisPoints exposes …SalesOrder via DiscountCeiling.PurityProbe"); restored
Guard classes (one at a time, all green)	AdapterReachRealTreeTests 2, PeerContractRealTreeTests 1, ModuleContractRealAssemblyTests 1, ContractDerivationTests 1, ModuleLedgerRealTreeTests 2, CouplingMatrixRealTreeTests 3, ModuleEdgeAnalyzerTests 14, NamespaceFolderAgreementTests 10, AdapterTierRealTreeTests 3, AccessOwnershipClaimTests 2, InsightsReadOnlyTests 25, TenantBypassRealTreeTests 4, SeamSurfaceRealAssemblyTests 2, TableOwnerRealModelTests 2, CompatibilityExceptionRealTreeTests 2
Affected unit tests	DiscountCeilingTests 38, SalesOrderCeilingTests 16, AccountSettingsTests 39, UpdateFarmSettingsValidatorTests 62
Build	dotnet build Cluckwork.slnx with repo defaults: 0 warnings, 0 errors

Five now-unused using Cluckwork.Domain.Modules.Commerce.Sales; lines were removed: each removal was kept only if the solution still built. The Farm → Commerce edge reason in Modules/Farm.cs now names the type's new place; the edge's symbols are unchanged.