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/persistence/mod.rs

@@ -26,6 +26,7 @@ use crate::project::{DATA_DIR, HISTORY_LOG, PROJECT_YAML};
 // during rebuild (ADR-0015 §7) are re-exported below.
 mod csv_io;
 mod history;
+pub mod migrations;
 mod yaml;
 
 pub(crate) use csv_io::{decode_cell, parse_csv};
@@ -212,6 +213,15 @@ impl Persistence {
         let line = history::format_record(command_text, history::utc_iso8601_now());
         history::append(&path, &line)
     }
+
+    /// Read the most-recent `max_n` sources out of
+    /// `history.log` for input-history hydration on project
+    /// open (ADR-0015 §12). Returned in chronological order
+    /// (oldest first). A missing file is `Ok(Vec::new())`.
+    pub fn read_recent_history(&self, max_n: usize) -> Result<Vec<String>, PersistenceError> {
+        let path = self.project_path.join(HISTORY_LOG);
+        history::read_recent_sources(&path, max_n)
+    }
 }
 
 /// Write `body` to `path` atomically via temp file + fsync +