Lights: Slice F — Shadow controls (directional cascaded, spot, point)

[fernandotonon]

Parent epic: #482
Depends on: Slice C

Goal

Per-light shadow casting with sensible defaults and Ogre's existing shadow techniques. Keeps the no-shadow path free for users who don't want shadows.

Scope

• Per-light Cast Shadows toggle in the Light inspector.
• Per-light Receive Shadows toggle on materials/entities (existing Ogre concept — surface via the inspector for entities).
• Directional shadows: cascaded shadow maps via Ogre::PSSMShadowCameraSetup. Configurable cascade count (2/3/4, default 3) and split lambda. One global config for now — per-light overrides are stretch.
• Spot shadows: standard shadow mapping. Resolution selectable (512/1024/2048).
• Point shadows: cubic shadow mapping (or omni-directional via 6 spots if the cube path is messy in our Ogre version).
• Global Shadow Quality preset: Off / Low / Medium / High — maps to shadow map resolution + filter quality + cascade count.
• Bias controls: depth bias + slope-scaled bias, exposed in an advanced expander per light (defaults that work for most scenes).
• New ShadowController (src/ShadowController.{h,cpp}, QML_SINGLETON) holds global shadow params; per-light overrides on the LightManager side.
• Apply shadow technique lazily on first light enabling shadows; uninstall when no lights cast shadows (keeps no-shadow path zero-cost).

Acceptance Criteria

• Each light type can cast a shadow that visibly affects the scene.
• No shadow performance cost when no light has shadows enabled (verified by frame-time check on a fixture scene).
• Shadow Quality presets produce visibly different shadow softness/resolution.
• Cascaded directional shadows handle a 50m scene with reasonable quality at default settings.
• Bias controls eliminate visible acne on a default-lit plane.
• Cross-platform: works on macOS arm64, Windows MinGW, Linux CI.
• Sentry breadcrumb scene.light.shadow_toggle per light.

Effort

~7 days. Highest technical risk in the epic, mostly platform/driver quirks.