feat(collector): capture additional smartctl diagnostics

Record host read and write commands, controller busy time, error log
entries, warning and critical temperature time, and thermal management
transitions from the existing smartctl -a -j acquisition. Schema 7 adds
nullable columns to samples so legacy rows read as unknown, and the
Diagnostics panel shows each counter with its change.

src/fenris/store.py

@@ -11,7 +11,23 @@ from typing import Optional
 
 
 # Schema version - increment on each migration
-SCHEMA_VERSION = 6
+SCHEMA_VERSION = 7
+
+# Diagnostic counters added in schema 7 (issue #111), named after the
+# NVMe health-log keys they come from. All nullable: legacy rows and drives that
+# do not report a counter stay unknown rather than zero.
+DIAGNOSTIC_SAMPLE_COLUMNS = (
+    "host_reads",
+    "host_writes",
+    "controller_busy_time",
+    "num_err_log_entries",
+    "warning_temp_time",
+    "critical_comp_time",
+    "thm_temp1_trans_count",
+    "thm_temp1_total_time",
+    "thm_temp2_trans_count",
+    "thm_temp2_total_time",
+)
 
 
 # Packaged default placement (spec §8.3). The config may override it, but a
@@ -109,7 +125,17 @@ def _create_schema(conn: sqlite3.Connection):
             bytes_read INTEGER,
             critical_warning INTEGER,
             segment_id INTEGER,
-            local_tz TEXT
+            local_tz TEXT,
+            host_reads INTEGER,
+            host_writes INTEGER,
+            controller_busy_time INTEGER,
+            num_err_log_entries INTEGER,
+            warning_temp_time INTEGER,
+            critical_comp_time INTEGER,
+            thm_temp1_trans_count INTEGER,
+            thm_temp1_total_time INTEGER,
+            thm_temp2_trans_count INTEGER,
+            thm_temp2_total_time INTEGER
         )
     """)
     
@@ -303,6 +329,7 @@ def _apply_migrations(conn: sqlite3.Connection, current_version: int):
         4: _migrate_3_to_4,
         5: _migrate_4_to_5,
         6: _migrate_5_to_6,
+        7: _migrate_6_to_7,
     }
     while current_version < SCHEMA_VERSION:
         target_version = current_version + 1
@@ -408,6 +435,22 @@ def _migrate_5_to_6(conn: sqlite3.Connection) -> None:
     repair_legacy_local_day_evidence(conn)
 
 
+def _migrate_6_to_7(conn: sqlite3.Connection) -> None:
+    """Add nullable diagnostic counters to samples (issue #111).
+
+    Additive only: existing rows are untouched and read NULL (unknown).
+    Rollback (SQLite 3.35+, with the collector stopped): for each column in
+    DIAGNOSTIC_SAMPLE_COLUMNS run ``ALTER TABLE samples DROP COLUMN <name>``,
+    then ``PRAGMA user_version=6``. Dropping loses only the new counters.
+    """
+    cols = {row[1] for row in conn.execute("PRAGMA table_info(samples)").fetchall()}
+    if not cols:
+        return
+    for column in DIAGNOSTIC_SAMPLE_COLUMNS:
+        if column not in cols:
+            conn.execute(f"ALTER TABLE samples ADD COLUMN {column} INTEGER")
+
+
 def migrate_to_latest(store_path: Path) -> int:
     """Apply forward-only migrations to bring the store to SCHEMA_VERSION.