feat: clear_table — empty the roster on a table change

"Clear the table" had no tool behind it, so she claimed she did it and nothing
changed. Add clear_table (empties the roster, keeps the session/stack/reads) and
a `replace` flag on seat_players for a one-shot table swap. Cash card: on a table
change / "clear the table", call clear_table then seat the new table — never claim
it without calling the tool.

2 tests. Full suite green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyra/poker.py

@@ -1888,6 +1888,20 @@ def unseat_player(name: str | None = None, descriptor: str | None = None,
     return True
 
 
+def clear_roster(session_id: int | None = None) -> int:
+    """Empty the table roster (he changed tables) — unseat everyone at once. Keeps
+    the session and any reads logged; just resets who's currently seated. Returns
+    how many were cleared."""
+    sid = _resolve(session_id)
+    if sid is None:
+        return 0
+    conn = _c()
+    with conn:
+        cur = conn.execute(
+            "UPDATE session_players SET active = 0 WHERE session_id = ? AND active = 1", (sid,))
+    return cur.rowcount
+
+
 def session_roster(session_id: int | None = None) -> list[dict]:
     """The live table roster: seated players with seat, dossier, and their latest
     read this session. This is 'who's at the table right now'."""