Add iOS typing perf automation and profiling workflows

[futurepaul]

Summary

This PR adds an iOS typing-performance workflow focused on composer keypress responsiveness and UI-thread work during typing.

It introduces:

• A dedicated iOS UI perf test suite for typing latency in TypingPerfUITests.
• New just one-liners for summary, trace capture, keypress stack analysis, and samply profiling.
• Symbolized Time Profiler attach-mode analysis that reports per-keypress averaged stack hotspots and writes flamegraph artifacts.
• App-side keypress signposts in ChatView to correlate text changes with sampled work.
• Simulator/Xcode runtime selection improvements to avoid destination/runtime mismatch failures.
• nix shell additions for inferno and samply.
• A new guide: docs/perf-testing.md.

New commands

• just ios-typing-perf
  • Prints one-line key findings (avg, best, worst, samples, plus bulk baseline when available).
• just ios-typing-perf-trace
  • Captures/open Time Profiler trace while running typing perf.
• just ios-typing-perf-keypress-profile
  • Exports per-keypress averaged stacks + folded output.
• just ios-typing-perf-keypress-profile-symbolized
  • Attach-mode symbolized stack collection + flamegraph SVG.
• just ios-typing-perf-samply
  • Attempts samply attach profiling for app PID.
  • Includes attach retries and local codesigning of nix-store samply copy.
  • Falls back to symbolized xctrace flow by default when samply hits known attach instability.

Why fallback exists

On macOS simulator attach, samply can intermittently panic with:

• Could not check process libraries
• InvalidAddress

To keep the one-liner reliable for day-to-day perf diagnosis, the script falls back to ios-typing-perf-keypress-profile-symbolized unless explicitly disabled.

Scope / current limitations

• This is focused on typing-path instrumentation and profiling mechanics.
• The test currently opens note-to-self chat and measures composer typing.
• It does not yet include a synthetic 10k-message fixture load path.

Validation done

• bash -n sanity checks for all added/updated shell tooling.
• End-to-end runs of:
  • ios-typing-perf
  • ios-typing-perf-keypress-profile-symbolized
  • ios-typing-perf-samply (including fallback behavior)

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