adventure: T6 Inversion Stitch (Seamstress) + boss re-strengthen (P8)

Second in-combat Layer-2 mechanic, reusing Amendment's round-end seam. In the
Seamstress's phase 2 (<=35% HP) the room sews inside-out in a repeating
warn(1)->sting(2) cadence, telegraphed one round ahead: during a pulse player
heals (self + ally) invert to damage, floored at 1 HP. State
inversionActive/inversionTelegraph round-trips through CombatStatuses so a
suspend/resume can't drop or double a pulse.

Sim surfaced that unplace is reach-bound (only ~37% reach the boss, then cleared
her ~98%), so the mechanic alone is sub-noise. Re-strengthened the Seamstress,
the weakest T6 boss (over-softened by P7 for a zone lift that never came):
HP 385->460, Atk 39->45, Needle Rain proc 0.40->0.45. Sim-validated at
fighter zone 37.5% (in range), and a real boss fight now instead of a victory lap.

Claude-Session: https://claude.ai/code/session_0156WqjgsbmSY2U8eQ3Kkb1s

internal/plugin/combat_engine.go

@@ -452,6 +452,16 @@ type combatState struct {
 	enemyRewindHP   int
 	enemyRewindUsed bool
 
+	// Inversion Stitch (T6 Seamstress) — an in-combat Layer-2 hook resolved at
+	// round end by applyBossInCombatRoundEnd, live only in the boss's phase 2.
+	// inversionActive is the number of rounds the room stays sewn inside-out
+	// (heals sting instead of mend, gated in stepPlayerActionEffect); it counts
+	// down one per round end. inversionTelegraph warns the round before a pulse
+	// activates, so a player who watches the tell can hold their heals. Both
+	// round-trip through CombatStatuses; zero/false for every non-Seamstress fight.
+	inversionActive    int
+	inversionTelegraph bool
+
 	round  int
 	events []CombatEvent