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.

src/app.rs

@@ -248,6 +248,27 @@ impl App {
         }
     }
 
+    /// Replace the in-memory navigable history with `entries`,
+    /// truncating to the in-memory cap.
+    ///
+    /// Used by the runtime to hydrate from the project's
+    /// `history.log` on open (I2-persist, ADR-0015 §12).
+    /// Entries should arrive in chronological order (oldest
+    /// first); the most recent stays at the back, which is
+    /// where Up/Down navigation expects it.
+    ///
+    /// Cancels any in-flight history navigation so a hydrate
+    /// during a session (e.g. after `load`) doesn't leave a
+    /// dangling cursor pointing at a now-removed entry.
+    pub fn seed_history(&mut self, entries: Vec<String>) {
+        self.history = entries;
+        while self.history.len() > HISTORY_CAPACITY {
+            self.history.remove(0);
+        }
+        self.history_cursor = None;
+        self.history_draft = None;
+    }
+
     /// Effective mode for the *next* submission, given the
     /// persistent mode and the current input buffer. See
     /// [`EffectiveMode`].
@@ -353,12 +374,14 @@ impl App {
             AppEvent::ProjectSwitched {
                 display_name,
                 is_temp,
+                history_entries,
             } => {
                 self.note_system(format!("[ok] now editing: {display_name}"));
                 self.project_name = Some(display_name);
                 self.project_is_temp = is_temp;
                 self.tables.clear();
                 self.current_table = None;
+                self.seed_history(history_entries);
                 Vec::new()
             }
             AppEvent::ProjectSwitchFailed { error } => {