fix(transition): crossfade backgrounds in float and complete the pan - #131
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Three defects in the camera pan, all measured. The background stepped by a full level across the whole frame at every junction. `set_alpha_f` quantises to an 8-bit alpha; while that byte is below 255 the premultiplied blend truncates ~1 LSB per channel everywhere, and the instant it reaches 255 Skia takes the exact opaque path and every pixel gains the level back in a single frame. The step measured exactly 1.0 in all six transitions of the dark-premium template, and a prediction that it moves to wherever the alpha byte first saturates held on four different easings. Crossfading in f32 removes it: 1.01 to 0.01 at the same frame, now matching the surrounding curve. `background: static` held the outgoing scene's background for the entire pan and then cut to the incoming one — 37.9 luminance units in a single frame on a synthetic pair. It now crossfades, which is a no-op when the two backgrounds match and softens the cut when they do not, rather than erroring on a case authors can legitimately write. Transitions never rendered their completed state: frames ran 0..N-1 against a divisor of duration*fps, so progress peaked at 0.978 and the residual was discharged in the first normal frame. Measured as a 22px foreground snap with linear easing; now 0px. Pixel baselines shift for every camera_pan, and slightly for other transitions near their last frame — both intended. Closes #124
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Closes #124. Part of the chantier #123.
Independent of the other six workstreams: disjoint file set, compiles and tests green on its own against
main. Mergeable in any order.Full rationale, measurements and evidence are in the commit message and in #124.