docs: the OpenMap zoom ladder end to end, and carrying the tile mode to an existing save

A default only reaches a fresh install, and grom's OpenMapClassic.lua
pins minimapTileMode = "fdid" -- the one value that cannot work on a
client with no FileDataIDs. OpenMapClassic has a migration hook for
exactly this, so the port edit adds a one-shot flip beside its own six,
flagged so an explicit /omc tilemode fdid afterwards is respected.

Measured through the real wheel handler: thirty clicks out reaches
0.0330, the ladder's floor and the whole world; forty back in reaches
2.3734, its ceiling; 27 steps between. Captured at zoom 1, 3, 9 and 40
with his own settings and no setup chunk.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SgstFWtn1RxEJ5kVrpgYwn

docs/ADDON_COMPAT.md

@@ -2990,3 +2990,28 @@ open something or create a frame and still have the scene settle around it. `$WO
 keeps the shutter, where a read and the pixels it explains come from the same frame. The deep-zoom
 shot needed the first: the tile mode has to be switched before anything draws, and a chunk at the
 shutter is a frame too late to appear in the image.
+
+### Carrying the switch to a save that already exists
+
+The default only reaches a fresh install. grom's `OpenMapClassic.lua` pins
+`["minimapTileMode"] = "fdid"`, and a saved value wins over a default, so the change would never
+have reached the one person it was made for. OpenMapClassic has a migration hook for exactly this
+(`Core.lua:255` calls `mod.Migrate(saved)` before the defaults fill), so the port edit adds one
+more entry beside its own six: a one-shot flip of `fdid` to `twm`, flagged so an explicit
+`/omc tilemode fdid` afterwards is respected. Verified: his saved file reads `twm` after a load.
+
+### The ladder, end to end
+
+Driven through `WorldMapButton`'s own wheel handler, thirty clicks out and forty back in:
+
+```
+start         scale=1.0000
+30 wheel out  scale=0.0330   <- the ladder's floor, the whole world
+40 wheel in   scale=2.3734   <- its ceiling
+ladder 0.0330 .. 2.3734 over 27 steps
+```
+
+Both ends reachable, which is what "scroll all the way out to the world map and all the way in to
+the minimap" asks for. Captured at zoom 1, 3, 9 and 40 with grom's own settings and no setup
+chunk: the world sheet with both continents, Eastern Kingdoms with its zones, Dun Morogh with its
+neighbours stitched in, and the minimap's terrain drawn through the map.