Add per-character spellbook collector

New Collectors/Spellbook.lua walks GetNumSpellTabs() and emits one entry
per tab with name, texture, offset, numSpells, and a spells[] array of
{ slot, name, rank, spellID, icon }.

3.3.5/Ascension API differences are handled defensively: name comes
from GetSpellBookItemInfo or vanilla GetSpellName; spellID is parsed
out of GetSpellLink (3.3.5 doesn't return it from GetSpellBookItemInfo);
icon falls back to GetSpellTexture if GetSpellInfo doesn't have it yet.

Wiring:
  - CoaExporter.toc: load Collectors/Spellbook.lua after Talents
  - Core.lua: AssembleExport() includes spellbook for all + spellbook
  - Core.lua: HandleExport() accepts /coae export spellbook
  - Core.lua: debug output shows tab + spell counts
  - UI/ExportFrame.lua: "Spellbook" button in the Character section

README.md

@@ -44,8 +44,9 @@ Primary slash: `/coae`. Aliases: `/coaexp`, `/ascx`, `/asxc`.
 ### Per-character export
 
 ```
-/coae export all              JSON of talents + gear + mystic enchants
+/coae export all              JSON of talents + spellbook + gear + mystic enchants
 /coae export talents          JSON, talents only
+/coae export spellbook        JSON, full spellbook (per tab, with spellID + rank + icon)
 /coae export gear             JSON, gear only
 /coae export enchants         JSON, mystic enchants only
 
@@ -127,6 +128,7 @@ CoaExporter/
 ├── Data/ScrollCatalog.lua          (auto-generated from AtlasLootAscension)
 ├── Collectors/                      (per-character data)
 │   ├── Talents.lua
+│   ├── Spellbook.lua
 │   ├── Gear.lua
 │   ├── Enchants.lua
 │   ├── MysticScrolls.lua