combat: revive the caster sustained-cantrip floor in the turn engine

The arcane-blaster "sustained floor" passives (CantripPerRound, DamageBonus,
FlatDmgStart from casterBlasterFloor) were built for the swing-based engine
(SimulateCombat, combat_engine.go:590). But every live expedition auto-resolves
through the turn engine (autoDriveCombat -> session -> combat_turn_engine),
where casters autocast every turn and never weapon-swing -- so CantripPerRound
never fired and DamageBonus was inert. Casters fought at bare cantrip dice
(~4d10~=22 at L20) instead of their intended floor, in sim AND in prod. This is
why every caster damage dial read as a dead lever across the whole rebaseline.

Fix (combat_cmd.go): bridge the already-computed CantripPerRound into the
turn-engine damage-cantrip cast, hit-gated (only lift a cast that already
connected, so the ~35% miss variance survives and the floor isn't a guaranteed
flat hammer). Self-targeting: only Mage/Sorcerer/Warlock carry a nonzero
CantripPerRound -- martials swing (untouched), cleric/bard/druid have floor 0.

Tuning (dnd_passives.go): casterCantripBase 9 -> 3, now a live, class-specific
lever. Mage/Sorcerer take base 3; Warlock passes 0 (its bare-dice cantrip plus
a structural edge already lands it mid-band, so an added floor overshoots).
Removed the dead casterHPPerLevel rider (it inflated the truncation-fraction
denominator without adding startable HP -- a bug).

Also lands the deterministic-seeding infra (sim_seed.go + simIntN/simFloat64
threading) used to read these deltas out of the process-seed noise; prod is
byte-identical (unseeded -> package rand).

Confirmation (expedition-sim, L20 T5 dragons_lair+abyss_portal, n=250):
casters now in the 35-45 floor -- sorcerer 39, mage 38, warlock 36; martial
leaders undisturbed (rogue 68, druid 66, ranger 65, fighter 64, ... paladin 55).

internal/plugin/dnd_rest.go

@@ -2,7 +2,6 @@ package plugin
 
 import (
 	"fmt"
-	"math/rand/v2"
 	"sort"
 	"strings"
 	"time"
@@ -117,7 +116,7 @@ func (p *AdventurePlugin) handleDnDShortRest(ctx MessageContext) error {
 	before := c.HPCurrent
 	if !hpFull {
 		conMod := abilityModifier(c.CON)
-		healDie := 1 + rand.IntN(6) // 1d6
+		healDie := 1 + simIntN(6) // 1d6
 		heal := healDie + conMod
 		if heal < 1 {
 			heal = 1