D&D: import Open5e SRD spell lists, make the five casters playable

Rebuilds the cmd/open5e-import CLI (fetch/gen × spells) to vendor
data/open5e/spells.json (319 SRD spells) and generate
dnd_spells_srd_data.go (237 after the level>5 filter). mapClasses unions
the API's incomplete structured spell_lists field with the complete
free-text dnd_class field so all eight casters get spells.

dndSpellRegistry loads buildSRDSpellList() first; the hand-authored
buildSpellList() overlays it (hand wins on ID collision). Playable=true
flipped for Druid/Bard/Sorcerer/Warlock/Paladin, each with a
defaultKnownSpells case. TestDefaultKnownSpellsExistInRegistry now covers
all eight classes.

.gitignore: vendor data/open5e/ while keeping the rest of data/ ignored,
and anchor the open5e-import binary pattern so it stops swallowing the
cmd/open5e-import source dir. NOTICE adds CC-BY-4.0 / SRD attribution.

internal/plugin/dnd_spells_test.go

@@ -171,7 +171,10 @@ func TestSpellAttackBonus(t *testing.T) {
 // ── Default known list ──────────────────────────────────────────────────────
 
 func TestDefaultKnownSpellsExistInRegistry(t *testing.T) {
-	for _, class := range []DnDClass{ClassMage, ClassCleric, ClassRanger} {
+	for _, class := range []DnDClass{
+		ClassMage, ClassCleric, ClassRanger,
+		ClassDruid, ClassBard, ClassSorcerer, ClassWarlock, ClassPaladin,
+	} {
 		for _, lvl := range []int{1, 3, 5, 9, 13, 20} {
 			for _, sid := range defaultKnownSpells(class, lvl) {
 				if _, ok := lookupSpell(sid); !ok {