libpredict: predict incoming heals for built-in click casting (#44)

Built-in click casting drives the cast through secure frame attributes,
bypassing the CastSpell/CastSpellByName/UseAction hooks that populate
spell_queue -- so the heal-prediction lookup in the SPELL_START_SELF
handler missed and no incoming heal showed.

Populate spell_queue from the cast's own rank-specific spellId (via
C_Spell.GetSpellName/GetSpellSubtext) right before the cache lookup, so
prediction works regardless of how the cast was initiated. Deriving the
rank from the actual spellId also fixes stale-rank lookups when a prior
keybind cast left a different rank of the same spell queued.

libs/libpredict.lua

@@ -168,6 +168,9 @@ pfUI.libdebuff_spell_start_self_hooks["libpredict"] = function(spellId, casterGu
     return
   end
 
+  spell_queue[1] = spellName
+  spell_queue[2] = spellName .. (C_Spell.GetSpellSubtext(spellId) or "")
+
   if spell_queue[1] == spellName and cache[spell_queue[2]] then
     local amount   = cache[spell_queue[2]][1]
     local casttime = castTime