checkpoint for the demo. almost!!

Signed-off-by: Sienna Meridian Satterwhite <sienna@r3t.io>

crates/libmarathon/src/persistence/migrations/005_tombstones.sql

@@ -0,0 +1,13 @@
+-- Migration 005: Add tombstones table
+-- Stores deletion tombstones to prevent resurrection of deleted entities
+
+CREATE TABLE IF NOT EXISTS tombstones (
+    entity_id BLOB PRIMARY KEY,
+    deleting_node TEXT NOT NULL,
+    deletion_clock BLOB NOT NULL,
+    created_at INTEGER NOT NULL
+);
+
+-- Index for querying tombstones by session (for future session scoping)
+CREATE INDEX IF NOT EXISTS idx_tombstones_created
+ON tombstones(created_at DESC);