D&D: class-balance Phase 3 — off-tier caster lift (druid + sorcerer)

Design steer from user — "relatively easy but not too easy" — narrowed the
target: lift the embarrassing caster trailers on off-tier cells (a casual
player walking into a slightly too-hard dungeon underleveled) without
pushing the already-saturated in-tier ceiling.

Levers:
- Druid passive: was the only chassis with a purely defensive passive
  (5% DR, no offense), and it read it — L1/T1 mean 0.77 (lowest at the
  entry tier), L1/T2 0.04. Added a level + WIS-scaled FlatDmgStart burst,
  same shape as the Phase-2 Bard/Mage/Warlock pass. Kept the DR; no
  DamageBonus rider so high-tier ceilings stay flat.
- Sorcerer passive: burst base 3→5. Sorcerer was second-worst caster
  off-tier (L1/T2 0.10 vs Mage 0.27 pre-tune) despite a comparable stat
  line; the bump pulls it toward arcane-chassis parity.

Observed lifts:
- Druid L1/T1: 0.77 → 0.86 (+9pp) — chassis now functional at its
  intended tier
- L2/T2 cross-class spread: 77pp → 63pp; druid trailer 0.23 → 0.35
- L1/T1 spread: 23pp → 14pp

Off-tier diagnostic: added a focused log to TestClassBalance_Phase1_FullMatrix
that names the trailing class at each off-tier (lvl, tier) cell. Not
asserted — L1 in T2 is *supposed* to be hard, so the diagnostic is for
watching the gap, not the absolute number.

In-tier parity assertion (35pp band on the diagonal) still passes;
TestApplyClassPassives updated for the new druid/sorcerer FlatDmgStart
values; full plugin -short suite clean.

internal/plugin/dnd_xp_test.go

@@ -229,13 +229,16 @@ func TestApplyClassPassives(t *testing.T) {
 		// burst now also scales with level, and Warlock picked up +1 attack.
 		// CHA/INT are 0 in this zero-value sheet → abilityModifier = -5,
 		// clamped to 0 by clampNonNeg. Paladin at L1 is still 4 + 0.
+		// Phase 3 class-balance: Druid picked up a WIS-scaled FlatDmgStart burst
+		// (lvl 1 + clamp(mod(WIS=0)) = 1), and Sorcerer's burst base went 3→5
+		// (5 + 1 + clamp(mod(CHA=10)=0) = 6).
 		{ClassRogue, 0.05, 0, true, 0, 1.0, 0, 0},
 		{ClassMage, 0.05, 1, false, 0, 1.0, 1, 0},
 		{ClassCleric, 0, 0, false, 5, 1.0, 0, 0},
 		{ClassRanger, 0.05, 1, false, 0, 1.0, 0, 0},
-		{ClassDruid, 0, 0, false, 0, 0.95, 0, 0},
+		{ClassDruid, 0, 0, false, 0, 0.95, 1, 0},
 		{ClassBard, 0.05, 1, false, 0, 1.0, 1, 1},
-		{ClassSorcerer, 0.05, 0, false, 0, 1.0, 4, 0},
+		{ClassSorcerer, 0.05, 0, false, 0, 1.0, 6, 0},
 		{ClassWarlock, 0.12, 1, false, 0, 1.0, 1, 0},
 		{ClassPaladin, 0, 0, false, 0, 1.0, 4, 0},
 	}