Improve initial load speed

[Cameron7195]

Summary
Reduce initial load time when opening large JSONL files in the extension.

Problem
Initial open of large files currently takes ~1 second or more, which feels sluggish compared to VS Code’s native text viewer (tens of milliseconds). Users expect near-instant feedback when opening files. * this is actually constant regardless of the file size and seems to have to do with the extension.

Proposed Solution

• Audit the extension’s startup pipeline:
  • Activation, provider creation, WebView creation, HTML generation, JS initialization, initial messaging, etc.
• Defer expensive work:
  • Lazy-load tokenization, Markdown rendering, and heavy parsing until needed or when a row becomes visible.
• Minimize HTML/JS payload:
  • Avoid giant HTML strings; consider modularizing or reducing DOM size.
• Add simple performance logging (dev mode) to identify bottlenecks.

Acceptance Criteria

• Opening a large test JSONL file is subjectively “snappy” (<300–400 ms on a typical machine) the first time.
• Profiling shows no single step is egregiously slow.
• Lazy behavior is confirmed (e.g., token counts or markdown are not computed until requested/visible).

[Cameron7195]

Ideally <100ms to feel basically instant regardless of file size as a reach goal

[neelbread]

@claude review this issue, ultrathink through the problem, then come up with an optimized solution to cut latency. Build in a separate branch, test the implementation, then ONLY once you have satisfied the requirement, create a PR for us to review