Add overlay extraction for multi-expansion asset deduplication

Extracts each expansion's assets as a CRC-compared overlay against a
base manifest, storing only files that differ. Auto-detects overlay mode
when a base manifest already exists. Adds --as-overlay, --full-base
flags and manifest merge for partial extractions.

Data/expansions/classic/db/TaxiPath.csv

@@ -286,3 +286,57 @@
 496,84,87,0
 499,53,44,830
 500,44,53,1020
+967,300,24,0
+1221,348,196,0
+1500,0,0,0
+1501,502,503,0
+1600,500,501,0
+1601,19,182,0
+1602,18,183,0
+1603,182,19,0
+1604,183,18,0
+1605,67,184,330
+1606,184,67,300
+90,175,176,50
+91,176,175,50
+92,175,7,50
+93,7,175,50
+94,176,6,50
+95,6,176,50
+1613,39,185,1020
+1614,185,39,1020
+1615,40,185,1020
+1616,185,40,1020
+1617,77,180,330
+1618,180,77,330
+1619,508,509,0
+1621,52,186,1020
+1623,53,186,1020
+1624,186,53,1020
+1625,186,52,1020
+1626,14,187,330
+1627,187,14,330
+1628,13,188,330
+1629,188,13,330
+1631,197,28,330
+1632,28,197,330
+1633,509,508,0
+1636,301,302,0
+1637,510,511,0
+1638,511,510,0
+1639,11,189,330
+1640,189,11,330
+1641,6,190,110
+1642,190,6,110
+1644,82,2,50
+1645,2,82,50
+1646,83,4,110
+1647,4,83,110
+1650,88,20,330
+1651,20,88,330
+1652,89,40,330
+1653,40,89,330
+1654,90,17,330
+1655,17,90,330
+1656,8,91,330
+1657,91,8,330