feat(camera): camera-handoff blend prototype (cam_blend)

[noctonca]

Prototype for smoothing the camera jump cuts, with the full design in
docs/CAMERA_BLEND_DESIGN.md.

The problem

LBA2's exterior camera is driven by several controllers (dynamic follow,
scripted zone cameras that pull back for a scenic view, cutscenes, manual
recenter) that each grab the camera by hard-writing its globals. Every handoff
is an instant jump cut: entering a scenic zone cuts, leaving it cuts back, and
Center camera snaps.

Phase A (this PR)

A blend layer at the single camera commit point (before AffScene) watches the
committed camera params. When they jump beyond a threshold (a handoff), it
presents an interpolation from the previous frame's camera toward the new target
over cam_blend_dur frames: RegleTrois for position and distance, shortest-arc
for the angles, weight-blended toward the live target so a moving dynamic follow
converges cleanly rather than aiming at a stale point.

• Catches the in-scene zone-in, zone-out, and manual-recenter handoffs with one
  integration point and no changes to the controllers.
• Scene transitions (NumCube change, plus an 8-frame settle) and cutscenes
  (CinemaMode) are cut, not blended. Classic camera and interiors untouched.
• cam_blend (default on) / cam_blend_dur cvars, persisted to cfg.
• Verified inert with no handoff (byte-stable against a no-blend capture within
  the scene's animation-noise floor); the blend feel needs interactive testing
  (walk into a scenic camera zone).

Known rough edges (see the design doc)

This is a prototype. Divergence detection is a heuristic (a very fast legitimate
move could trip it; a tiny zone offset could miss it), the blend presents the
parametric camera without the per-frame occlusion lift (pops in at blend end),
and interpolation is linear (ease-in-out is a feel refinement). Instrumenting the
handoff sites directly and per-zone opt-out are the natural next steps.

Not in this PR

Phase B (a fully free dynamic camera: hold angle, re-center only on interior to
exterior + manual) and interior-to-exterior blending are separate, described in
the design doc.

How to test

Enable the Auto camera, walk into an area with a scripted scenic camera, and
watch the transition in and out. cam_blend 0 for the old jump cut; cam_blend_dur
to tune the length (frames).