feat(tickets): checkpoint backend V1 du système de tickets (T1-T5)

Checkpoint de travail — QA formelle encore à venir (après reset Codex).
`cargo build` workspace OK, tests ticket/issue verts (domaine, store,
use cases, app-tauri 47/51). Le domaine s'appelle `Issue`, exposé
« ticket » côté MCP/UI.

- T1 domaine : entité Issue (statut, priorité, liens, carnet),
  events, ids, ports.
- T2 infra : store FS Markdown + allocator de références #N.
- T3 application : use cases Issue (create/read/list/update/status/
  priority/carnet/link/unlink/assign) + erreurs dédiées.
- T4 surface MCP : 10 outils publics idea_ticket_* (23 outils au
  total), mappés vers les use cases Issue.
- T5 app-tauri : commandes Tauri ticket_* + DTOs d'events Issue +
  câblage state.rs.

Dette de test PRÉ-EXISTANTE héritée de develop (4 tests app-tauri
mcp_e2e_loopback / mcp_serve_peer rouges) hors périmètre tickets,
non traitée ici.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/domain/tests/issue.rs

@@ -0,0 +1,120 @@
+use std::str::FromStr;
+
+use domain::{
+    AgentIssueRef, AgentIssueRole, Issue, IssueActor, IssueError, IssueId, IssueLink,
+    IssueLinkKind, IssueNumber, IssuePriority, IssueRef, IssueStatus, IssueVersion, MarkdownDoc,
+};
+
+fn issue_number(n: u64) -> IssueNumber {
+    IssueNumber::new(n).unwrap()
+}
+
+fn issue() -> Issue {
+    Issue::new(
+        IssueId::new_random(),
+        issue_number(7),
+        "Fix boot reconcile",
+        MarkdownDoc::new("Description"),
+        IssueStatus::Open,
+        IssuePriority::Medium,
+        MarkdownDoc::new("Carnet"),
+        vec![IssueLink {
+            target: IssueRef::from(issue_number(8)),
+            kind: IssueLinkKind::RelatesTo,
+        }],
+        vec![AgentIssueRef {
+            agent_id: domain::AgentId::new_random(),
+            role: AgentIssueRole::Assigned,
+        }],
+        IssueActor::User,
+        1_000,
+    )
+    .unwrap()
+}
+
+#[test]
+fn issue_ref_parses_and_formats_strict_hash_number() {
+    let reference = IssueRef::from_str("#42").unwrap();
+
+    assert_eq!(reference.number().get(), 42);
+    assert_eq!(reference.to_string(), "#42");
+    assert_eq!(
+        IssueRef::from_str("42"),
+        Err(IssueError::InvalidRef("42".into()))
+    );
+    assert_eq!(IssueRef::from_str("#0"), Err(IssueError::InvalidNumber));
+    assert_eq!(
+        IssueRef::from_str("#abc"),
+        Err(IssueError::InvalidRef("#abc".into()))
+    );
+}
+
+#[test]
+fn issue_number_must_be_positive() {
+    assert_eq!(IssueNumber::new(0), Err(IssueError::InvalidNumber));
+}
+
+#[test]
+fn issue_title_must_not_be_empty() {
+    let err = Issue::new(
+        IssueId::new_random(),
+        issue_number(1),
+        "   ",
+        MarkdownDoc::new("Description"),
+        IssueStatus::Open,
+        IssuePriority::Medium,
+        MarkdownDoc::default(),
+        Vec::new(),
+        Vec::new(),
+        IssueActor::User,
+        1,
+    )
+    .unwrap_err();
+
+    assert_eq!(err, IssueError::EmptyTitle);
+}
+
+#[test]
+fn issue_rejects_self_link() {
+    let err = Issue::new(
+        IssueId::new_random(),
+        issue_number(9),
+        "Self link",
+        MarkdownDoc::new("Description"),
+        IssueStatus::Open,
+        IssuePriority::Medium,
+        MarkdownDoc::default(),
+        vec![IssueLink {
+            target: IssueRef::from(issue_number(9)),
+            kind: IssueLinkKind::Blocks,
+        }],
+        Vec::new(),
+        IssueActor::User,
+        1,
+    )
+    .unwrap_err();
+
+    assert_eq!(
+        err,
+        IssueError::SelfLink {
+            reference: IssueRef::from(issue_number(9))
+        }
+    );
+}
+
+#[test]
+fn issue_mutation_increments_version_and_updates_actor_time() {
+    let issue = issue();
+    assert_eq!(issue.version, IssueVersion::INITIAL);
+
+    let updated = issue
+        .mutate(IssueActor::System, 2_000, |i| {
+            i.status = IssueStatus::InProgress;
+        })
+        .unwrap();
+
+    assert_eq!(updated.version.get(), 2);
+    assert_eq!(updated.status, IssueStatus::InProgress);
+    assert_eq!(updated.updated_by, IssueActor::System);
+    assert_eq!(updated.updated_at, 2_000);
+}