Lights: Slice E — Light preset rigs + ambient/env controls

[fernandotonon]

Parent epic: #482
Depends on: Slice D

Goal

One-click preset rigs that produce believable lighting for the most common review/authoring scenarios. Replace the hard-coded default scene light with the "Three-point studio" rig.

Scope

• New LightRigLibrary singleton (in src/LightRigLibrary.{h,cpp}). Returns a parameterised set of lights for a named rig.
• Initial rigs:
  • Single Key — directional only, current default. Kept for the "minimum" case.
  • Three-point studio — key + fill + back, classic portrait lighting. Becomes the new default for CreateEmptyScene.
  • Outdoor sunset — strong warm directional + soft cool ambient. Pairs with the sunset_outdoor HDRI from HDR: Slice F — Bundled HDRI library + "HDR Environment" preset additions #472.
  • Outdoor overcast — soft directional, high ambient.
  • Indoor window — directional through a "window" + warm bounce. Pairs with indoor_window HDRI.
  • Product turntable — overhead key + two side fills, neutral colours, no shadows. For clean asset shots.
• GUI: "Add Light → Preset Rig…" submenu and a button in the new Lighting section of the Inspector.
• Applying a rig:
  • Adds the rig's lights as a group under a single named SceneNode ("Three-point rig", "Sunset rig", etc.).
  • Optionally replaces existing user lights (confirm dialog) or adds alongside them.
• Ambient + scene background controls in a new "Lighting" inspector group (or the existing Environment group from HDR slice E):
  • Ambient colour (replaces the per-scene mainwindow ambient picker if appropriate).
  • Background mode: solid colour, gradient, HDR skybox (linked to Epic: HDR — HDR lighting, IBL, and tone mapping #466).
• All applied as a single undo command (the entire rig is one atomic add).
• Replace the hard-coded line in Manager::CreateEmptyScene with LightRigLibrary::apply("three_point_studio") on first scene init; preserve "Single Key" as a setting fallback for users who prefer the old look.

Acceptance Criteria

• All six rigs apply cleanly and produce visibly distinct, believable lighting on a default sphere.
• Replacing existing lights goes through a confirm dialog.
• Rig application is a single undo step.
• New empty scenes use the three-point studio rig by default.
• User can revert to the single-key default via a setting.
• Sentry breadcrumb scene.light.apply_rig with the rig name.

Effort

~5 days.