unitframes/buffwatch/libpredict: route aura reads through C_UnitAuras

ClassicAPI's recent C_UnitAuras additions (sourceUnit / sourceGUID /
non-player expirationTime / the PLAYER filter token) finally cover
everything libdebuff:UnitDebuff and :UnitOwnDebuff were doing — caster
attribution, accurate timing for non-player units, and the own-debuffs
filter. Migrating the remaining callers off the libdebuff readers.

- api/unitframes.lua: collapses the debuff render path's three-branch
  if/else into a single C_UnitAuras.GetAuraDataByIndex with a HARMFUL
  or HARMFUL|PLAYER filter selected by the selfdebuff config; tooltip
  slot-finders match by sourceGUID instead of libdebuff's caster flag;
  custom-debuff indicator scan unifies the same way.
- modules/buffwatch.lua: GetBuffData drops the libdebuff fallback;
  tooltip slot-finder mirrors the unitframes pattern.
- libs/libpredict.lua: drops the orphaned UnitHasBuff slot-loop (no
  callers left).
- api/api.lua: pfUI.api.UnitHasBuff tightens from a HELPFUL iteration
  to a single GetAuraDataBySpellName lookup.

Each site that builds a cooldown ring from expirationTime carries the
talent-extension guard — when expirationTime exceeds the dbc base
duration (e.g. Shadow Affinity → SW:P), clamp start to now and use the
remaining time as the effective duration so CooldownFrame_SetTimer
doesn't get a future start it treats as "not yet begun".

api/api.lua

@@ -193,10 +193,7 @@ end
 -- name         [string]        The localized name of the buff.
 -- return:      [bool]          true if unit has buff otherwise "nil"
 function pfUI.api.UnitHasBuff(unit, name)
-  for _, aura in ipairs(C_UnitAuras.GetUnitAuras(unit, "HELPFUL")) do
-    if aura.name == name then return true end
-  end
-  return nil
+  return C_UnitAuras.GetAuraDataBySpellName(unit, name, "HELPFUL") ~= nil or nil
 end
 
 -- [ GetUnbuffedRoster ]