refactor: replace Prepare Generation code node with predefined nodes

The Prepare Generation code node in "[SWF] anime: process one image" did
nothing that needed scripting: measure the source image, pick an orientation
matched resolution, set a fixed prompt prefix and seed, and gate on the
safety analysis. Replaced with six predefined nodes wired off Safety Gate +
Prompt directly: an Analysis Present check that fails the run loudly via
Stop and Error if the payload is missing (the code node used to throw for
this; an expression would have silently resolved to empty and generated a
default square image instead), a Store Eligible gate, an Image node (info
operation) for measurement, and two Set Fields nodes for the not-eligible
defaults and the final generation parameters.

Verified field-for-field against the original code node across landscape,
portrait and square real source images plus the not-eligible, blocked and
missing-payload cases, using an isolated harness workflow that ran both
implementations side by side on the runner. The not-eligible path was also
proven three times end to end in live production. Old node kept disabled
and disconnected until proven, then removed. Published (publishedVersion
21) since call-workflow only runs published versions, and re-synced
deploy/workflows/swf-anime-process-one-image.json.