Expose and package temporal rendering with guarded edge history

docs/manual/editor/temporal.md

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+# Temporal rendering
+
+Faset renders the scene with **Off** by default. In the optional Editor diagnostics
+panel (**F12**), choose **TAA** to accumulate a full-resolution scene over successive
+frames, or **Upscale** to render the scene at a lower resolution and reconstruct it
+at the output resolution. The Upscale slider accepts 50–99%; 67% is a useful
+starting point for visual comparison. UI text and controls always render at output
+resolution after the scene resolve. Shadow maps keep their own unjittered views.
+
+The diagnostic selector affects only the current Editor viewport. It does not edit
+the scene, gameplay code, or an exported Player. The panel's **Requested** and
+**Effective** fields identify a device fallback. It also shows internal and output
+extent, whether the previous completed frame's color history was eligible, the
+reason it reset, and separate GPU times for resolve, composite and UI where
+timestamp queries are available. A reset on the first frame, camera cut, changed
+view, resize, scale switch or compatible shader reload is expected. A valid history
+does not imply every pixel reused it: newly visible surfaces can still reject
+their individual history samples.
+
+For a Player or exported game, select the mode at launch:
+
+```sh
+./faset_player --headless --frames 120 --temporal taa --profile taa.json
+./faset_player --headless --frames 120 --temporal upscale \
+  --render-scale 0.67 --profile upscale.json
+```
+
+`--temporal` accepts `off`, `taa`, or `upscale`. Off and TAA use scale `1`; Upscale
+requires a scale from `0.5` inclusive to `1` exclusive. An invalid mode or scale
+stops startup with an error. If Vulkan compute or the required image formats are
+unavailable, the renderer falls back to Off and records its effective mode and
+reason in the profile. Direct, GPU frustum and GPU occlusion visibility can be
+combined with either temporal mode. See [Profiling](profiling.md) for how to compare
+their timings fairly.
+
+Native renderer users can make the same choice without modifying gameplay scripts:
+
+```cpp
+faset::render::RendererConfig config;
+config.temporal_mode = faset::render::TemporalMode::Upscale;
+config.render_scale = 0.67f;
+faset::render::Renderer renderer(config);
+
+// A live viewport switch recreates scene targets and resets color history.
+renderer.set_temporal_mode(faset::render::TemporalMode::TAA);
+```
+
+Provide a stable `DrawItem::instance_key` for moving opaque objects so the renderer
+can find their previous model transform. Camera cuts must be marked in the
+`Snapshot`; cuts, teleports and incompatible projection changes reject old history.
+World transparency and sprites use the scene depth/order and reject stale color on
+their reactive pixels. TAA and Upscale are optional image-quality paths; compare
+them against Off on the actual game scene, especially thin geometry, slow pans,
+newly revealed surfaces and moving transparent content.