fix(encode): derive png-seq frame index from task position - #136
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The output index came from a shared atomic incremented in rayon's completion order rather than from the frame's position, so frames landed out of order within each batch. Two runs of the same input differed on 114 frames, and multi-threaded output was a strict permutation of single-threaded output with 436 of 486 frames misplaced. It raised the measured noise floor 9.4x and manufactured spikes that corrupted an analysis before being caught. The index is now batch_base + local position. Rendering stays fully parallel — only the index computation changed. Two multi-threaded runs and a single-threaded run now produce byte-identical sequences, cross-checked against the --frame N path on three sample frames. mp4 and gif were never affected and are unchanged. Closes #129
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Closes #129. 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 #129.