Upstream PackedLayout computes a pin's time coordinate in two branches and
raises for anything between them. They are the two endpoints of one line:
the packed sequence advances FRAME_RESCALE per PIXEL frame, so
cond_t = text_len + FRAME_RESCALE * pixel_index reproduces both exactly at
every clip length, including the irregular (1,4,4,4,4) latent grouping.
Upstream still builds the layout; this rewrites the time column of the
condition rows it produced. Endpoint-only runs call straight through and are
bit-identical. The rule is re-derived from upstream's own constants before
patching, so a changed frame grid disables the feature instead of misplacing
pins.
Unlocks mid-clip guides on the Chain Sampler, which were refused outright.
EXPERIMENTAL: fl2va trained on first/last pins only, so the coordinate is
representable without being learned. Costs no extra model call.
digital-garbage
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