Seam Craft — render prerequisites for scene-to-scene transitions

This is the render-correctness doctrine for PLV scene-to-scene seams: the prerequisites and master-timeline mechanics that make any transition composite correctly, independent of which specific transition is chosen. The per-transition catalog (crossfade, push-slide, zoom-through, cut-the-curve, …) lives in the transition registry — this page is the doctrine that sits underneath all of them.

The transitions this doctrine governs are Tier-B-ready: pure transform / opacity / filter on the two scene clip wrappers (#el-<sid>), no injected overlay DOM, no per-scene cooperation. Overlay families (staggered blocks, blinds, light leak, grid dissolve, page burn) and shader transitions are deferred to later phases.

Stage ground prerequisite (white-flash guard)

Several templates open a window where the two wrappers' summed opacity < 1 (the cut-the-curve mid-window cut, zoom-through's 0.15 floor, plain crossfade's power-curve dip). Whatever is BEHIND the wrappers shows through during that window. If the assembled index.html #root has no opaque background, the renderer composites the dip over its default white page → a white flash at every seam, glaring on dark films (observed on two Spotify runs before the fix). The assembler must paint the stage: #root { background: var(--canvas-deep, var(--canvas, #000)) } — assemble-index.mjs now emits this; any other consumer of these templates owns the same guarantee.

How the injector applies a transition

At a break boundary between scene i (from) and scene i+1 (to), the injector:

1. Extends #el-<from> wrapper data-duration by duration_s (holds its final frame — verified: core/src/runtime/init.ts:1393-1410 external-slot branch).
2. Pulls #el-<to> wrapper data-start earlier by duration_s (creates the overlap window).
3. Reassigns all clip data-track-index as a 0/1 ping-pong so the two overlapping wrappers never share a track (same-track overlap is illegal — core/src/lint/rules/composition.ts). Higher track composites on top.
4. Stamps the gsap_template into window.__timelines["main"] at T = overlap-start.

Verified by prototype render (2026-05-31): the master-timeline wrapper tween is seeked and rendered (no double-seek with the sub-comp's own paused timeline — the runtime drives them independently), the extended wrapper holds scene i's final frame, and the higher-track incoming wrapper composites over + blends with the outgoing one.

Template placeholders

The injector substitutes these tokens in each gsap_template line:

filter / scaleX / transformOrigin are lint-clean on the master timeline (verified: core/src/lint/rules/gsap.ts has no per-property whitelist and scopes its checks to data-composition-id ranges; the x/y/scale/rotation/opacity whitelist is a scene-worker prompt rule only — it does not bind index.html).