fix(core): splitElementInHtml data-end + text-content inner wrapper

[vanceingalls]

What

Three independent correctness fixes in packages/core/src/studio-api/helpers/sourceMutation.ts, extracted as a standalone PR so they can merge without waiting for the R1 stack.

1. setElementDuration — stale sibling attribute

When writing data-end, the old code left data-duration untouched (and vice versa). An element that previously used data-duration and got split would have both attributes after the split, with the runtime reading whichever it found first.

// now removes the other attr in each branch:
if (usesDataEnd) {
  el.setAttribute("data-end", endTime);
  el.removeAttribute("data-duration");  // ← added
} else {
  el.setAttribute("data-duration", duration);
  el.removeAttribute("data-end");       // ← added
}

2. text-content op — inner wrapper breadth

The text-content patch targets firstElementChild to avoid clobbering outer element structure. The old check required tagName === "div"; the generator can wrap text in any single-child element (<p>, <span>, etc.).

// before: only unwrapped div wrappers
// after: unwraps any single-child element
const inner = htmlEl.children.length === 1 ? htmlEl.firstElementChild : null;

3. data-playback-rate — NaN default

parseFloat on a missing attribute returns NaN. The trim offset calculation would produce NaN media start times for any element without an explicit data-playback-rate.

const rateRaw = parseFloat(el.getAttribute("data-playback-rate") ?? "");
const rate = Number.isFinite(rateRaw) ? rateRaw : 1;  // default 1× when absent/malformed

Why

All three are correctness bugs found during code review of the R1 stack. They exist independently of R1 and are safe to merge at any time.

Test plan

• Existing sourceMutation.test.ts split tests cover setElementDuration behavior
• Full test suite passes (380 tests, 0 fail)

[vanceingalls]

• fix(core): splitElementInHtml data-end + text-content inner wrapper #1273 👈 (View in Graphite)
• main

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[miguel-heygen]

Closing — all three fixes (setElementDuration stale sibling, text-content inner wrapper, data-playback-rate NaN guard) have been folded into the gesture-to-keyframes branch (PR #1256). They'll land with that stack.