casts: route everything through C_Spell, drop libcast + libdebuff_casts

ClassicAPI's recent C_Spell additions cover remote-unit casts natively
(SMSG_SPELL_START co-hook caching per caster GUID), so the two parallel
cast trackers pfUI was running — libcast.lua and the libdebuff_casts
table inside libdebuff.lua — can both retire.

Migrations:
- modules/castbar.lua: focus/player cast-info gathering reads
  C_Spell.UnitCastingInfo / UnitChannelInfo directly. Fallback ladder
  (libdebuff_casts → pfGetCastInfo → pfGetChannelInfo) collapses into a
  single call. Pushback handlers stop writing back into a non-existent
  cache; the local this.endTime is the source of truth.
- modules/nameplates.lua: GetCastInfo(guid) now resolves to a unit token
  via UnitTokenFromGUID and queries C_Spell, returning the same compact
  struct shape downstream code expected. UpdateCastbar collapses from a
  three-branch hierarchy (dead IterDebuffs / libdebuff_casts / libcast)
  to one C_Spell read.
- modules/afkcam.lua: pfGetCastInfo+pfGetChannelInfo round-trip becomes
  a single C_Spell.UnitCastingInfo("player") or UnitChannelInfo fallback.
- libs/libpredict.lua: HealComm timing uses C_Spell on the sender's unit
  token after a small group-roster walk to resolve the sender's name.

Deletions:
- libs/libcast.lua entirely (-571 lines) plus its init/libs.xml entry.
- libdebuff_casts / libdebuff_item_icons tables and their write sites
  in libs/libdebuff.lua (the SPELL_START_*, SPELL_GO_*, SPELL_FAILED_*
  event handlers stop maintaining them but keep firing the
  libdebuff_*_hooks broadcast surface for actionbar / swingtimer /
  libtotem). SPELLCAST_CHANNEL_STOP now reads the active channel from
  C_Spell.ChannelInfo.
- modules/superwow.lua's supercast block — UNIT_CASTEVENT writes into
  libcast.db are redundant now that C_Spell co-hooks the same packet.
- The cast-bar item-icon override that swapped in a potion/trinket
  icon for item-triggered casts. Spell icon stays; the item-icon
  metadata path (libdebuff_item_icons) went with libdebuff_casts.

Steady Shot synthetic cast bar — Turtle WoW-specific:
- castbar.lua gains a pfUI.synthetic_casts[unit] fallback that fires
  only when C_Spell returns nil, so abilities the engine treats as
  instant but which have a meaningful wait window can still render a
  cast bar.
- modules/turtle-wow.lua replaces the old libcast.customcast block with
  a Nampower SPELL_QUEUE_EVENT subscriber. ON_SWING_QUEUED matching
  the localized "Steady Shot" name writes a 1.4s synthetic entry;
  ON_SWING_QUEUE_POPPED clears it; castbar's endMs guard self-expires
  the entry as a safety net. Note: haste scaling (libcast.ApplyShotHaste)
  is gone — bar may finish slightly early under +ranged haste buffs.

Net: 152 insertions, 972 deletions.

libs/libpredict.lua

@@ -546,30 +546,47 @@ function libpredict:ParseComm(sender, msg)
           rank = tonumber(rankStr)
         end
       end
-    elseif select and pfGetCastInfo then
+    elseif select then
       -- latest healcomm
       msgtype = tonumber(string.sub(msg, 1, 3))
       if not msgtype then return end
 
+      -- Resolve sender's name to a group unit token so C_Spell can query
+      -- the cast. Group rosters are tiny (44 slots max) so the walk is cheap
+      -- relative to the cost of receiving a HealComm message.
+      local function senderUnit()
+        if UnitName("player") == sender then return "player" end
+        for i = 1, GetNumPartyMembers() do
+          if UnitName("party"..i) == sender then return "party"..i end
+        end
+        for i = 1, GetNumRaidMembers() do
+          if UnitName("raid"..i) == sender then return "raid"..i end
+        end
+      end
+
       if msgtype == 0 then
         msgtype = "Heal"
         heal = tonumber(string.sub(msg, 4, 8))
         target = string.sub(msg,9, -1)
 
-        local starttime = select(5, pfGetCastInfo(sender))
-        local endtime = select(6, pfGetCastInfo(sender))
-        if not starttime or not endtime then return end
-        time = endtime - starttime
+        local unit = senderUnit()
+        if not unit then return end
+        local startMs = select(4, C_Spell.UnitCastingInfo(unit))
+        local endMs = select(5, C_Spell.UnitCastingInfo(unit))
+        if not startMs or not endMs then return end
+        time = (endMs - startMs) / 1000
       elseif msgtype == 1 then
         msgtype = "Stop"
       elseif msgtype == 2 then
         msgtype = "Heal"
         heal = tonumber(string.sub(msg,4, 8))
         target = {strsplit(":", string.sub(msg,9, -1))}
-        local starttime = select(5, pfGetCastInfo(sender))
-        local endtime = select(6, pfGetCastInfo(sender))
-        if not starttime or not endtime then return end
-        time = endtime - starttime
+        local unit = senderUnit()
+        if not unit then return end
+        local startMs = select(4, C_Spell.UnitCastingInfo(unit))
+        local endMs = select(5, C_Spell.UnitCastingInfo(unit))
+        if not startMs or not endMs then return end
+        time = (endMs - startMs) / 1000
       end
     end
   end