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.

modules/turtle-wow.lua

@@ -312,42 +312,41 @@ pfUI:RegisterModule("turtle-wow", function ()
     end
   end)
 
-  -- add Trueshot recognition to to custom castbars.
-  if not libcast.customcast["steadyshot"] then
-    -- add trueshot to pfUI's custom casts
-    local player = UnitName("player")
+  -- Synthetic Steady Shot cast bar. The engine treats Steady Shot as
+  -- instant, but the arrow doesn't actually fire until the next ranged
+  -- swing (~1.4s baseline). Drive a synthetic entry off Nampower's
+  -- SPELL_QUEUE_EVENT so hunters see a tickdown matching the swing wait.
+  -- castbar.lua reads pfUI.synthetic_casts[unit] as a fallback when
+  -- C_Spell.UnitCastingInfo returns nil.
+  pfUI_locale["enUS"]["customcast"]["TRUESHOT"] = "Steady Shot"
+  pfUI_locale["zhCN"]["customcast"]["TRUESHOT"] = "稳固射击"
+  pfUI.synthetic_casts = pfUI.synthetic_casts or {}
 
-    -- add locales
-    pfUI_locale["enUS"]["customcast"]["TRUESHOT"] = "Steady Shot"
-    pfUI_locale["zhCN"]["customcast"]["TRUESHOT"] = "稳固射击"
-    local trueshot = L["customcast"]["TRUESHOT"] or ""
+  local steadyShotName = L["customcast"]["TRUESHOT"]
+  local STEADY_SHOT_ICON = "Interface\\Icons\\Ability_hunter_steadyshot"
+  local STEADY_SHOT_DURATION_MS = 1400  -- not haste-scaled; close enough for the visual cue
+  local ON_SWING_QUEUED, ON_SWING_QUEUE_POPPED = 0, 1
 
-    libcast.customcast[strlower(trueshot)] = function(begin, duration)
-      if begin then
-        -- cast time is 1sec, however it takes 1.4sec to fire in average
-        local duration = libcast.ApplyShotHaste(duration or 1400)
-
-        local _,_, lag = GetNetStats()
-        local start = GetTime() + lag/1000
-
-        -- add cast action to the database
-        libcast.db[player].cast = trueshot
-        libcast.db[player].rank = lastrank
-        libcast.db[player].start = start
-        libcast.db[player].casttime = duration
-        libcast.db[player].icon = "Interface\\Icons\\Ability_hunter_steadyshot"
-        libcast.db[player].channel = nil
-      else
-        -- remove cast action to the database
-        libcast.db[player].cast = nil
-        libcast.db[player].rank = nil
-        libcast.db[player].start = nil
-        libcast.db[player].casttime = nil
-        libcast.db[player].icon = nil
-        libcast.db[player].channel = nil
-      end
+  local steadyShot = CreateFrame("Frame")
+  steadyShot:RegisterEvent("SPELL_QUEUE_EVENT")
+  steadyShot:SetScript("OnEvent", function()
+    if event ~= "SPELL_QUEUE_EVENT" then return end
+    local eventCode, spellId = arg1, arg2
+    if eventCode ~= ON_SWING_QUEUED and eventCode ~= ON_SWING_QUEUE_POPPED then return end
+    if C_Spell.GetSpellName(spellId) ~= steadyShotName then return end
+    if eventCode == ON_SWING_QUEUED then
+      local now = GetTime() * 1000
+      pfUI.synthetic_casts["player"] = {
+        name    = steadyShotName,
+        icon    = STEADY_SHOT_ICON,
+        startMs = now,
+        endMs   = now + STEADY_SHOT_DURATION_MS,
+        spellID = spellId,
+      }
+    else
+      pfUI.synthetic_casts["player"] = nil
     end
-  end
+  end)
 
   -- add skin to twow's talent inspect frame
   if pfUI.skin["Inspect"] and pfUI_config["disabled"]["skin_Inspect"] ~= "1" then