Refactor nameplate cast tracking to be fully GUID-based

Problem:
Since a recent Turtle-WoW update, nameplates would show the same castbar on all mobs sharing a name (e.g. multiple "Defias Rogue Wizard"). Additionally, a targeted mob that was not casting would incorrectly display the castbar of a nearby mob with the same name.

Changes:
**`modules/nameplates.lua`**
- Removed `hasNampower` version-sniffing entirely. Nampower is a hard requirement for pfUI; all related guards have been removed and the code runs unconditionally.
- Castbar lookup now always uses `nameplate.cachedGuid` directly — never `GetUnitGUID("target")` or a unit name string.
- Removed dead fallback block using `UnitCastingInfo("target")`.
- Restored fallback via `pfGetCastInfo(cachedGuid)` / `pfGetChannelInfo(cachedGuid)` (see libcast changes below) for casts not tracked by libdebuff (e.g. already-in-progress casts on login).

**`libs/libcast.lua`**
- Renamed `UnitCastingInfo` → `pfGetCastInfo` and `UnitChannelInfo` → `pfGetChannelInfo` to avoid confusion with the Blizzard API of the same name, which does not exist in Vanilla.
- Name-based fallback lookup (`libcast.db[unitName]`) is now skipped when a GUID is available. This prevents cast bleed between mobs that share a name.

**`libs/libpredict.lua`, `modules/castbar.lua`, `modules/afkcam.lua`**
- Updated all call sites to use the renamed `pfGetCastInfo` / `pfGetChannelInfo`.

modules/afkcam.lua

@@ -170,8 +170,8 @@ pfUI:RegisterModule("afkcam", "vanilla:tbc", function ()
     if ( this.tick or 0) > GetTime() then return else this.tick = GetTime() + 1 end
 
     local name = UnitName("player")
-    local cast = UnitCastingInfo(name)
-    if not cast then cast = UnitChannelInfo(name) end
+    local cast = pfGetCastInfo(name)
+    if not cast then cast = pfGetChannelInfo(name) end
     if not this.delay then this.delay = 0 end
 
     if cast then