Show trustworthy local-day activity totals (#90)

Add local-day activity summaries derived from UTC hour observations using
the system timezone, with durable storage in a new local_days table.
The collector derives local-day read/write totals after UTC aggregation;
the TUI displays them with timezone, completeness state, and coverage.

Schema: bump SCHEMA_VERSION to 3, add local_days table (migration 2→3
is pure addition, idempotent, preserves newer-schema refusal).

src/fenris/store.py

@@ -12,7 +12,7 @@ from typing import Optional
 
 
 # Schema version - increment on each migration
-SCHEMA_VERSION = 2
+SCHEMA_VERSION = 3
 
 
 # Packaged default placement (spec §8.3). The config may override it, but a
@@ -193,6 +193,26 @@ def _create_schema(conn: sqlite3.Connection):
     """)
 
 
+    # Local-day activity summaries derived from UTC hour observations.
+    # Each row retains its recorded timezone and UTC boundaries so that
+    # historical summaries survive a system-timezone change (ADR 0010).
+    conn.execute("""
+        CREATE TABLE IF NOT EXISTS local_days (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            local_date TEXT NOT NULL,     -- e.g. "2026-09-01" in the recorded tz
+            tz_name TEXT NOT NULL,        -- POSIX tz name, e.g. "Asia/Kolkata"
+            tz_offset TEXT NOT NULL,      -- e.g. "+05:30"
+            utc_start TEXT NOT NULL,      -- ISO 8601 UTC: local midnight start
+            utc_end TEXT NOT NULL,        -- ISO 8601 UTC: local midnight end
+            bytes_written INTEGER DEFAULT 0,
+            bytes_read INTEGER DEFAULT 0,
+            coverage REAL DEFAULT 0.0,
+            sample_count INTEGER DEFAULT 0,
+            complete BOOLEAN DEFAULT 0,
+            UNIQUE(local_date, tz_name)
+        )
+    """)
+
     # Metadata table for store state (e.g., legacy import marker)
     conn.execute("""
         CREATE TABLE IF NOT EXISTS store_metadata (
@@ -230,6 +250,31 @@ def _apply_migrations(conn: sqlite3.Connection, current_version: int):
                 conn.execute("ALTER TABLE day_aggregates ADD COLUMN unattributed_bytes_read INTEGER DEFAULT 0")
         current_version = 2
 
+    # Migration 2→3: add local_days table for local-day activity totals
+    # (issue #90, ADR 0010).  Pure addition — no existing rows touched.
+    if current_version < 3:
+        tables = {row[0] for row in conn.execute(
+            "SELECT name FROM sqlite_master WHERE type='table'"
+        ).fetchall()}
+        if "local_days" not in tables:
+            conn.execute("""
+                CREATE TABLE IF NOT EXISTS local_days (
+                    id INTEGER PRIMARY KEY AUTOINCREMENT,
+                    local_date TEXT NOT NULL,
+                    tz_name TEXT NOT NULL,
+                    tz_offset TEXT NOT NULL,
+                    utc_start TEXT NOT NULL,
+                    utc_end TEXT NOT NULL,
+                    bytes_written INTEGER DEFAULT 0,
+                    bytes_read INTEGER DEFAULT 0,
+                    coverage REAL DEFAULT 0.0,
+                    sample_count INTEGER DEFAULT 0,
+                    complete BOOLEAN DEFAULT 0,
+                    UNIQUE(local_date, tz_name)
+                )
+            """)
+        current_version = 3
+
 
 def migrate_to_latest(store_path: Path) -> int:
     """Apply forward-only migrations to bring the store to SCHEMA_VERSION.