Iteration 6: --resume + persistent input history + migration scaffold

Closes out track 2's ADR-0015 backlog.

* `--resume` CLI flag (L1a, ADR-0015 §7) opens the most-
  recently-used project, tracked in <data-root>/last_project.
  Mutually exclusive with a positional <project-path>; errors
  cleanly to stderr (above the shell prompt) on missing file
  or stale recorded path. last_project is rewritten on every
  successful project open (startup, load, new, save as,
  import).
* Persistent input history (I2-persist, ADR-0015 §12). On
  project open, the in-memory navigable history is hydrated
  from the tail of history.log (capped at the in-memory cap).
  ProjectSwitched gains a `history_entries` payload field;
  App::seed_history is the entry point. Pipes inside source
  text round-trip via splitn(3); unknown escape sequences are
  passed through literally.
* Migration framework scaffold (F3, ADR-0015 §9). New
  persistence::migrations module with MigratorRegistry +
  migrate_to_latest + ensure_project_yaml_migrated. Empty
  in v1 (production registry has no migrators); the loader
  runs through it on every project open and is exercised by
  tests with a fake v1→v2 migrator. Writes
  project.yaml.v<N>.bak before any migrator runs; verifies
  each step bumps the version field.

Refreshes docs/requirements.md (A1 / I2 / F3 / E1 / L1a /
test baseline) and adds docs/handoff/20260508-handoff-3.md
covering both Iter 5 and Iter 6.

Total tests: 408 passing, 0 failing, 0 skipped (up from 345
at handoff-2). Clippy clean.

tests/iteration4b_lifecycle_commands.rs

@@ -273,6 +273,7 @@ fn project_switched_event_updates_state() {
     app.update(AppEvent::ProjectSwitched {
         display_name: "New Name".to_string(),
         is_temp: false,
+        history_entries: Vec::new(),
     });
     assert_eq!(app.project_name.as_deref(), Some("New Name"));
     assert!(!app.project_is_temp);