feat: Fenris persistent TUI monitoring redesign

Implement the complete redesign per fenris-redesign spec:

- Observation store: SQLite WAL mode, six entities, schema versioning
- Collector: smartctl acquisition, sysfs identity, normalization
- Projection: sustained regime rate, habit change, confidence states
- Panes TUI: Textual keyboard-first layout with four normative regions
- Status CLI: read-only composition with four service facts
- Monitor helper: polkit-guarded toggle, collect, baseline ops
- Legacy migration: idempotent single-transaction import
- Hour classification, day aggregates, monitoring periods
- Pruning, segmentation, drive health facts

Cross-cutting acceptance sweep (CI-1 through CI-4):
- 59 tests covering state matrix, TUI/CLI parity, prohibition set,
  required wording and six disclosures
- Full suite: 289 tests, all green

Issues #20, #32 closed.

src/fenris/store.py

@@ -176,12 +176,53 @@ def _create_schema(conn: sqlite3.Connection):
 
 
 def _apply_migrations(conn: sqlite3.Connection, current_version: int):
-    """Apply forward-only migrations from current_version to SCHEMA_VERSION."""
-    # Future migrations will go here
-    # For now, just upgrade to current version
+    """Apply forward-only migrations from current_version to SCHEMA_VERSION.
+    
+    Each migration step is a transactional block. Add new steps as sequential
+    elif branches when SCHEMA_VERSION increases.
+    
+    Spec: §3.6, §10.2
+    """
+    # Migration 1→2: example placeholder
+    # if current_version < 2:
+    #     conn.execute("ALTER TABLE ...")
+    #     current_version = 2
     pass
 
 
+def migrate_to_latest(store_path: Path) -> int:
+    """Apply forward-only migrations to bring the store to SCHEMA_VERSION.
+    
+    Called by the upgrade target (§10.2). Returns the number of migration
+    steps applied. Raises ValueError on newer-schema store (§3.6, §9.5).
+    
+    Spec: §3.6, §10.2, §10.3
+    """
+    conn = sqlite3.connect(str(store_path))
+    conn.execute("PRAGMA journal_mode=WAL")
+    
+    cursor = conn.execute("PRAGMA user_version")
+    current_version = cursor.fetchone()[0]
+    
+    if current_version > SCHEMA_VERSION:
+        conn.close()
+        raise ValueError(
+            f"Observation store written by a newer Fenris (version {current_version}) "
+            f"— upgrade Fenris"
+        )
+    
+    if current_version == SCHEMA_VERSION:
+        conn.close()
+        return 0  # Already up to date
+    
+    steps = SCHEMA_VERSION - current_version
+    _apply_migrations(conn, current_version)
+    conn.execute(f"PRAGMA user_version={SCHEMA_VERSION}")
+    conn.commit()
+    conn.close()
+    return steps
+
+
 def is_store_faulty(store_path: Path) -> bool:
     """Check if the store is present but cannot be read or trusted."""
     if not store_path.exists():