Fix/enforce archive state as terminal

[amodelandme]

Summary

Flag now treats archival as a terminal state. Every mutation method on the entity
(SetEnabled, UpdateStrategy, UpdateName, Update, Archive) carries a guard
clause as its first statement: if IsArchived is true, the method throws a new
FlagDomainException whose message names the flag. The exception extends
BanderasException with StatusCode = 409, so the existing GlobalExceptionMiddleware
maps it to a 409 Conflict ProblemDetails response with no middleware change. The
change is contained entirely within the Domain layer — no controllers, services,
repositories, or migrations were touched.

Changes in this PR

• Domain — new FlagDomainException (sealed, BanderasException-derived, 409); guard clause added as the first statement of all five Flag mutation methods
• Tests — new FlagArchivedInvariantTests (10 tests: 5 archived-guard + 5 happy-path baselines)
• Docs — current-state.md, roadmap.md, and the Flag DDD backlog updated; spec + implementation notes added under Docs/Decisions/enforce-archive-state-as-terminal - PR# 59/

Spec

Docs/Decisions/enforce-archive-state-as-terminal - PR# 59/spec.md

Implementation Notes

Docs/Decisions/enforce-archive-state-as-terminal - PR# 59/implementation-notes.md

Definition of Done

• FlagDomainException exists in Banderas.Domain/Exceptions/ and extends BanderasException with StatusCode = 409
• Guard clause is the first statement in SetEnabled()
  (SetEnabled, UpdateStrategy, UpdateName, Update, Archive) carries a guard
  clause as its first statement: if IsArchived is true, the method throws a new
  FlagDomainException whose message names the flag. The exception extends
  BanderasException with StatusCode = 409, so the existing GlobalExceptionMiddleware
  maps it to a 409 Conflict ProblemDetails response with no middleware change. The
  change is contained entirely within the Domain layer — no controllers, services,
  repositories, or migrations were touched.

Changes in this PR

• Domain — new FlagDomainException (sealed, BanderasException-derived, 409); guard clause added as the first
  statement of all five Flag mutation methods
• Tests — new FlagArchivedInvariantTests (10 tests: 5 archived-guard + 5 happy-path baselines)
• Docs — current-state.md, roadmap.md, and the Flag DDD backlog updated; spec + implementation notes added
  under Docs/Decisions/enforce-archive-state-as-terminal - PR# 59/