Unify document viewer experience

[naaiyy]

Summary

• replace format-specific controls with one accessible, Pierre-based viewer control system
• add a shared viewer stage and fix DOCX fitting, PowerPoint rail scrolling, thumbnail resource bounds, and safe slide centering
• bound playground preview and Markdown panes and expand the PowerPoint test fixture to 10 slides
• add the new public viewer UI package to the lockstep alpha release system and centralize release package inventory

Why

The viewers had duplicated control APIs and inconsistent layout ownership. Search reserved empty space, DOCX could overflow its host at normal zoom, PowerPoint navigation and slide placement were unstable, and the playground panes could grow with their content.

This refactor makes the publishable renderer packages own those behaviors. The playground now only hosts and demonstrates them.

Breaking alpha API change

Consumers must replace renderControls and showDefaultControls with controls and onControls.

Validation

• complete build and TypeScript checks for all workspace packages
• 135 tests passed
• viewer bundle and ingestion performance budgets passed
• packed server and umbrella package closure checks passed
• release metadata and changeset status passed
• production dependency audit found no known vulnerabilities
• two-axis adversarial review completed; all hard findings and release risks were corrected