N3/P4: give every seat a row of its own

The turn engine seats a party since P3, but only seat 0 survived a
suspend: seats 1+ reopened from their Mods on every step, rearming their
once-per-fight one-shots -- a party Halfling rerolled a nat 1 every round.

Split CombatStatuses into the fight-scoped half (the enemy's stance, the
round cursor) and the per-character half, ActorStatuses. The embed is
anonymous and untagged, so statuses_json stays the same flat object it
always was and every in-flight prod row decodes unchanged.

Seat 0 keeps living on combat_session. Seats 1+ get combat_participant
rows. That asymmetry is the point: a solo fight -- which is every fight
that has ever run, and the whole balance corpus -- writes exactly the
bytes it wrote before, and no participant rows at all. roster_size
guards the read, so the solo loader never issues the second query.

Parties commit their seats in the same transaction as the session row;
solo keeps its single unwrapped UPDATE.

expedition_party is the co-op roster. No party_id on dnd_expedition:
expedition_id already identifies the party, and a second key would be a
second answer to "who is in this party". Absent means solo, in both new
tables, so neither needs a bootstrap backfill.

The combat characterization golden is byte-identical.

Also seeds and re-powers the two statistical subclass tests. They drew
from the package-global RNG, so their verdict depended on how much
randomness every test declared before them happened to consume -- which
is why they flaked on a clean tree. Sweeping 40 seeds: Precision Attack
had a mean margin of +127 wins against a +50 threshold but a worst case
of -42, and Assassinate averaged +12.8 with two seeds outright negative.
Both effects are real; the trial counts were too low to see them. Seeded,
and raised to 24000/6000 trials, where all 40 seeds clear.

internal/plugin/combat_session_build.go

@@ -132,16 +132,17 @@ func (p *AdventurePlugin) combatantsForSession(sess *CombatSession) (player Comb
 	// onto the freshly-rebuilt player. The depleting one-shots (ward/spore/…)
 	// live on the session's Statuses and flow through the turn engine's
 	// resume/commit cycle, so only the persistent stat deltas are applied here.
-	applySessionBuffs(&player, sess.Statuses)
+	applySessionBuffs(&player, sess.Statuses.ActorStatuses)
 	return player, enemy, err
 }
 
 // applySessionBuffs re-derives the persistent stat effect of every mid-fight
-// buff onto the rebuilt player. The buffs are stored as accumulated deltas
-// (diffTurnBuff produced them against the player's state at cast time), so
+// buff onto one rebuilt character. The buffs are stored as accumulated deltas
+// (diffTurnBuff produced them against that character's state at cast time), so
 // re-applying them to a deterministic rebuild reproduces the same totals every
-// round without double-counting.
-func applySessionBuffs(player *Combatant, s CombatStatuses) {
+// round without double-counting. It takes ActorStatuses rather than the whole
+// session because buffs are per-caster: each seat folds in only its own.
+func applySessionBuffs(player *Combatant, s ActorStatuses) {
 	player.Stats.AC += s.BuffACBonus
 	player.Stats.AttackBonus += s.BuffAtkBonus
 	player.Stats.Speed += s.BuffSpeedBonus