fix(core): handle track replacements in visual layout updates

[fatihcvs]

updatePages only adds new tracks when the total list length increases. If a track is replaced while another is removed (for example, switching the focused participant or replacing an unpublished camera with its placeholder), the swap searches for an item that was never added and throws Element not part of the array.

Always add the missing items before updating pages, preserving the existing page order and removal logic. Related to #524.

Validation:

• Four regression cases cover equal-size replacement, replacement while shrinking/growing, and camera-to-placeholder replacement. Three fail on the original code with the reported exception; all 43 sorting tests pass after the fix.
• 122 core/react/styles tests pass. pnpm build:react, core lint (14 existing warnings), pnpm format:check, and React API check pass.
• A separate check against the built core package covered 16,900 transitions of unique four-item subsets/permutations across page sizes 1–4, verifying exact output membership and unchanged inputs. No live meeting/browser call was used for this check.

Includes a core patch changeset. Developed and verified with OpenAI Codex assistance.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5fd64cc

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

Name	Type
@livekit/components-core	Patch
@livekit/components-react	Patch
@livekit/agents-ui	Patch
@livekit/component-example-next	Patch
@livekit/components-js-docs	Patch
@livekit/component-docs-storybook	Patch
@livekit/components-docs-gen	Patch

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