fix(ui-modal): avoid redundant Modal.Body re-renders from its observers

[balzss]

Summary

• Modal.Body's resize/mutation observers called forceUpdate() on every observed change, so any DOM change inside the body re-rendered it even when the derived tabIndex was identical. In jsdom test suites those updates land outside act(), which is what consumers on 11.7.4 are seeing.
• Derive needsTabIndex into state and compare against the last computed value before calling setState. The comparison has to precede setState — for class components it schedules (and warns) before the updater runs, so bailing out inside the updater is too late.
• render() no longer reads live DOM geometry; it reads state, and the DOM reads moved into syncTabIndex.
• Same change in v1 and v2. No prop, theme, or export changes.

Measured on the branch: 20 no-op mutations inside the body went from 20 re-renders to 0 (real Chromium), and 10 mutations from 10 act warnings to 0 (jsdom + RTL, both versions).

Test Plan

• Keyboard-only: open a modal with a scrollable body and no focusable children, Tab to the body, confirm it takes focus and Up/Down scroll it.
• With the modal still open, toggle a focusable child into the body — the body should stop being a tab stop and focus should go to the child instead. Toggle it back out and the body should be focusable again.
• Toggle the body between scrollable and not while open; the tab stop should follow.
• Worth a screen-reader pass (VO/NVDA/JAWS) on the scrollable-body aria-label, since this touches the same code path as INSTUI-5046.

Fixes INSTUI-5166

🤖 Generated with Claude Code

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Visual regression report

Cypress suite: ✅ Passing

Visual diff: ✅ No changes.

Status	Count
Unchanged	96
Changed	0
New	0
Removed	0

Accessibility (axe): ✅ No violations.

📊 View full report — click a screenshot's ⚠ badge to see each violation boxed on the image, with the offending element named and contrast failures shown as color swatches.

_{Baselines come from the visual-baselines branch. They refresh on every merge to master. The Cypress suite line covers the a11y and console-error assertions — a ❌ there means the suite found real issues even if the visual diff is clean.}