chore(db): switch meta.schema_version to monotonic semver (#888)

Replace SHA-1 hash in meta.schema_version with an orderable semver
string ("1.0.0"). SHA preserved in new schema_sha column for tamper
detection. Enables savegame migration lineage in Phase 5+ — saves can
record their schema version and determine which migrations to apply.

Updated both generators, check-systems-db-stamp validation (rejects
old SHA-hex values), schema DDL, and asset-pipeline docs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tooling/planet-gen/generate_atlas.py

@@ -92,6 +92,11 @@ _ATLAS_SCHEMA_END_MARKER = "-- END ATLAS INDEX"
 # Generator metadata stamp (#855, #856)
 # ---------------------------------------------------------------------------
 
+# Monotonic schema version — shared constant with import_economics.py (#888).
+# Bump manually on any backwards-incompatible schema change.
+SCHEMA_VERSION = "1.0.0"
+
+
 def _file_sha1(*paths: Path) -> str:
     """Return SHA-1 hex of the concatenated content of one or more files.
 
@@ -124,9 +129,9 @@ def _write_stamp(conn: sqlite3.Connection) -> None:
     generator_sha = _file_sha1(Path(__file__))
     conn.execute(
         """INSERT OR REPLACE INTO meta
-               (generator_name, schema_version, generator_sha, generated_at)
-           VALUES ('generate_atlas', ?, ?, datetime('now'))""",
-        (schema_sha, generator_sha),
+               (generator_name, schema_version, schema_sha, generator_sha, generated_at)
+           VALUES ('generate_atlas', ?, ?, ?, datetime('now'))""",
+        (SCHEMA_VERSION, schema_sha, generator_sha),
     )
 
 
@@ -172,11 +177,17 @@ def ensure_atlas_schema(conn: sqlite3.Connection) -> None:
         CREATE TABLE IF NOT EXISTS meta (
             generator_name TEXT PRIMARY KEY,
             schema_version TEXT NOT NULL,
+            schema_sha TEXT,
             generator_sha TEXT NOT NULL,
             generated_at TEXT NOT NULL DEFAULT (datetime('now'))
         );
         """
     )
+    # Add schema_sha column to existing DBs that pre-date #888 (#888 migration).
+    try:
+        conn.execute("ALTER TABLE meta ADD COLUMN schema_sha TEXT")
+    except Exception:
+        pass  # column already exists
 
 
 def _first_int(values, default: int = 0) -> int: