fix(ui): align transcript projection tests with timeline model

[liuxiaocs7]

Summary

After #5510 merged, CI and Windows recovery fail while compiling @maka/ui: four new transcript projection assertions access TurnViewModel.assistant, which #5559 removed.

Assert the ordered text segments in timeline instead. This preserves coverage for appended and replaced text, and for previous projections remaining immutable snapshots.

Refs #5510, #5559. Failure logs: CI, Windows recovery.

Verification

• Reproduced the four TS2339 errors with npm --workspace @maka/ui run typecheck before the fix.
• npm run build — passed.
• npm --workspace @maka/ui run test:dist — all 652 tests passed, including the affected projection tests.
• npm run lint and npm run typecheck — passed.
• Knip for apps/desktop and packages/ui — passed.
• Biome formatting across all Git-tracked files — passed. The full working-tree format command encounters unrelated untracked local files; they are not part of this PR. packages/ui is formatter-excluded by repository configuration.
• git diff --check — passed.

Windows recovery was not run locally (macOS); the existing Windows failure occurs in the shared TypeScript compilation step.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Codex diagnosed the CI failure, updated the assertions, ran verification, and prepared this PR at the contributor's request. The commit includes Generated-by: Codex.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[Astro-Han]

LGTM.