fix(world): stop the camera teleporting and the frame budget losing frames (#152)

Eight confirmed audit findings on scene transitions and the `world` view —
the one mechanism meant to read as a single continuous shot, and the one
where every defect below is plainly visible on screen.

- A transition longer than the scene it leaves silently deleted frames from
  the *entering* scene. The outgoing scene cannot spend more frames than it
  has, but the budget was computed as if it could, and the overrun was
  taken out of the next scene. Now clamped to the outgoing scene's own
  frame count, with the accounting asserted frame by frame.
- `camera_pan_duration` longer than a scene teleported the camera:
  measured jump of 245.33px between two consecutive frames, now 21.33px.
- The outgoing scene's background held at full strength through the whole
  pan, then vanished in one frame — an avg-luma step of 109.0 between two
  frames, now at most 4.82.
- The scene cross-fade drove the *whole* frame to 50% opacity at every pan
  midpoint (both scenes at 0.5, nothing behind them). Mid-pan opacity is
  now 0.670 instead of sitting on that floor.
- `persist: true` snapped opacity back to 1.0 while the scene was still
  fading out — a 0.326 discontinuity, now 0.034.
- A view-to-view transition duplicated the junction frame on both sides.
  `ViewTransition` progress is now an open interval, matching what
  `SlideTransition` already guaranteed.
- `scene.freeze_at` was read by no world code path at all.
- `scene.effects` (post-effects) were dropped on world frames and on view
  transition frames.

Every figure above is a before/after measurement from the tests added here,
not an estimate.

Tests: 144 pass on this branch alone.

Author: LeadcodeDev