fix(simulation): align vertical box drawing

[kitlangton]

What

Render the home-screen composer's heavy vertical border as cell-aligned geometry instead of a font glyph.

Before: Commit Mono's ┃ and ╹ glyph metrics could bleed across terminal cell boundaries, making the blue composer border look fuzzy, too tall, or overlapping at catalog scale.

After: ┃ is a crisp two-pixel full-cell rule and ╹ is its two-pixel upper half-cell cap, matching the intended OpenTUI box geometry.

How

• Extend SimulationPng.drawBlockElement with geometric ┃ and ╹ rendering.
• Add a pixel-level regression test covering the full-cell rule and half-cell cap.

Scope

This only changes screenshot rendering. Terminal layout and the captured frame protocol are unchanged.

Testing

• bun run typecheck in packages/simulation
• bun test in packages/simulation (20 tests)
• Repository pre-push typecheck across 32 packages
• Browser verification against the terminal catalog at native 10×20 cell geometry