Walking scenes consolidated; broken-bridge content lands

Walking scenes were three near-duplicate systems — hub walking, story walking, and walking-battle each had their own presentation layer. Today they consolidate into a single shared presentation, so adding a new walking scene is a content step rather than a custom-wiring step. Trigger enforcement landed at the same time: a hero can't pass through a trigger zone without firing it, so collisions actually catch.

Story content: broken-bridge — scene 5 of the demo arc — got its written content laid in. That moves the demo cut closer to having every placeholder scene replaced.

Party selector and battle popup got a round of fixes. Web layout was clipping panels at certain viewport widths; that's resolved. Enemies on the battle popup are now grouped per fight instead of dumped as a flat list, and the rewards block is centered so it reads as the focal point of the popup.