Add structured validation failures

docs/internal/overview.md

@@ -46,7 +46,7 @@ these implemented packages:
 | `internal/tempdir` | Disposable system temporary storage outside source worktrees. |
 | `internal/process` | Cancellable child-process execution with per-command environment overlays and removals. |
 | `internal/preparation` | Coordination for expected-commit tag checkout and Go module dependency preparation. |
-| `internal/validation` | Shared default Go validation for captured candidates; `internal/candidate` retains capture, callback sequencing, and local-state rechecks. |
+| `internal/validation` | Shared default Go validation and structured check failures for captured candidates; `internal/candidate` retains capture, callback sequencing, and local-state rechecks. |
 | `internal/publish` | Publication guards, one validation run, annotated tag creation, push, and recovery state. |
 | `internal/runtimeinfo` | Private version lookup used by the runtime facade. |
 | `internal/versioncontract` | Shared stable-version rules and linker-symbol identity. |

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Validation model implementation plan
 
-Status: In progress; the shared validation executor is complete.
+Status: In progress; the shared validation executor and internal diagnostics are complete.
 
 This plan implements the enabling work in [model.md](model.md), guided by
 [ADR 0003](../adr/0003-declarative-validation.md). It is written for a Luna
@@ -206,6 +206,10 @@ inventory to reflect the implemented executor.
 
 ## Stage 2: Introduce internal validation diagnostics
 
+**Completion:** Complete. Shared Go setup and test failures carry the internal
+`go-test` identity and preserve their underlying errors, including cancellation
+and subprocess exit errors.
+
 **Outcome:** The executor returns a small structured failure with an underlying
 cause, independently of public error conversion.