fix(core): stop autoplay on load + propagate seek to sibling timelines

[jrusso1020]

Follow-up to #359 for the async-scene-loading path — compositions served as raw static files with data-composition-src children (e.g. the Vercel template), as opposed to the bundleToSingleHtml studio path.

The bug

On page load, <hyperframes-player> reports the master as paused at 0:00 but each scene visibly animates — the "autoplay on load" bug. Traced via a paused-setter proxy on each registered timeline: on init, each scene's _ts flipped 0→1 inside ensureChildCandidatesActive in init.ts, calling timeline.paused(false) on every child.

That helper predates #359 and was the only way a user-click Play would animate scenes — back when play didn't propagate through the registry. After #359, createRuntimePlayer.play already iterates every registry entry and un-pauses each, so the init-time un-pause is redundant for user-driven playback. It still matters for the producer's render path, though: there the master's totalTime cascade is the only driver (no GSAP ticker — virtual time), and a paused child won't re-render on cascade, so every producer regression baseline depends on children being un-paused at init time.

Fix

Scope the un-pause to render mode, signalled by window.__HF_VIRTUAL_TIME__ (set by the producer's render-mode bootstrap). Preview mode leaves children paused — no autoplay — and play/pause still propagate via the registry iteration added in #359.

Known limitation (follow-up)

In preview mode, a scrub-back after a full playthrough can still leave async scenes parked at their end-state. Root cause is deeper: addMissingChildCandidatesToRootTimeline in init.ts isn't actually reparenting async scene timelines to the master (inspection shows scene._dp === gsap.globalTimeline instead of rootTimeline after the add call). GSAP's cascade skips paused non-children. Fixing that properly needs a separate change to the async composition loader and is tracked as a follow-up; the autoplay fix here is the narrow, safe piece.

Files

• packages/core/src/runtime/init.ts — add isRenderMode guard around the existing ensureChildCandidatesActive call
• packages/core/src/runtime/player.ts — unchanged behavior from fix(player+core): correctly render and pause nested compositions #359; seek/renderSeek intentionally do NOT iterate the registry (clearer JSDoc explains why — iterating would overwrite GSAP's cascade with wrong absolute times)
• packages/core/src/runtime/player.test.ts — 2 new tests documenting that seek/renderSeek don't iterate the registry

Reproducing the regression failures

This PR's previous revision removed ensureChildCandidatesActive unconditionally, which broke 25 PSNR checkpoints across overlay-montage-prod, style-4-prod, style-7-prod, style-8-prod, style-9-prod, style-17-prod, style-18-prod. Reproducing locally with:

docker build -f Dockerfile.test -t hyperframes-producer:test .
docker run --rm --security-opt seccomp=unconfined --shm-size=4g \
  -v "$PWD/packages/producer/tests:/app/packages/producer/tests" \
  hyperframes-producer:test style-17-prod

shows the same failures (would have caught it before push — lesson learned).

With the render-mode-scoped fix, all 4 tests I spot-checked run to 0/100 failed frames.

Testing

• cd packages/core && bunx vitest run — 490/490 pass (+2 new)
• cd packages/player && bunx vitest run — 35/35 pass (unchanged)
• bun run typecheck in core + player — clean
• bunx oxlint / bunx oxfmt --check clean across changed files
• Docker regression: style-17-prod 0/100, style-7-prod 0/100, style-4-prod 0/100, overlay-montage-prod 0/100
• Live Vercel deploy verified: scenes stay paused on load (autoplay fixed); play/pause propagates; render output unchanged