Element wheel reworked to a sequential chain; chains reenact on the field with banter portraits; 4.0.0 cut

Sequential element chain. The element structure is reworked into a single sequential chain across backend, app, and banter. One ordering drives the math, the spell-card visuals, and the banter relations — no more keeping parallel models in sync.

Chain reenactment on the field. When a chain resolves in combat, the chain plays out across the actual cells on the field, with the participating heroes' banter portraits leading the burst. The chain stops being a hidden bonus calculation; it's a visible beat tied to the cells involved.

Comic burst bubble redesign. The burst-bubble component gets a redesign: center-tail and no-tail variants, plus a leading-avatar slot. The new chain choreography uses it to hang the speaker portrait off the bubble.

Second peaceful music track. A second peaceful track lands in the music pool, so the calm rooms have more than one option to rotate through.

Version cut 4.0.0. Major version bump for the dev → staging promotion that carries the element rework.