Metal frame pacing: add CI-level rendering validation

[daiimus]

Description

The CADisplayLink + ghostty_surface_draw_now() integration (commit 9f2c6f7) was implemented for ProMotion 120Hz support but has never been validated. We need simulator-based CI checks that verify:

1. The display link fires at the expected cadence
2. Metal command buffers are actually submitted during rendering
3. Frame pacing is consistent (no excessive hitches)

Approach

Add a simulator-based test to ci.sh that:

• Launches Geistty in the simulator
• Sends rapid screen-updating content (ANSI escape sequences, scrolling output)
• Measures frame callback timing via #if DEBUG instrumentation
• Asserts frame intervals are within expected bounds

This should run as part of the existing ci.sh test or as a new ci.sh perf-test command. Simulator-only — no device required.

Context

• A Metal System Trace was captured on Icarus but only showed system compositor frames, not Geistty-specific rendering
• Manual visual testing is insufficient for frame pacing (60Hz vs 120Hz is hard to detect by eye)
• The display link code was never verified to actually call through to the Zig render path