Filter Instagram webview bridge errors

[zacjones93]

Summary

• add a narrow PostHog before_send filter for the Instagram iOS webview bridge exception
• preserve unrelated exceptions and all non-exception analytics events
• add regression coverage and document the client error-filtering boundary in lat.md

Root cause

The reported PostHog issue was produced by Instagram's injected iOS webview lifecycle bridge, which assumes window.webkit.messageHandlers exists. Because the inline script inherits the current document URL, PostHog marked the frames as application code even though the bridge functions are absent from WODsmith source and the production page.

PostHog issue: https://us.posthog.com/error_tracking/019f1f7e-2399-7a02-8e8e-9c845fbe6d4b

Validation

• pnpm --filter wodsmith-start exec vitest run test/lib/posthog-event-filter.test.ts
• file-scoped Biome check
• file-scoped TypeScript check
• lat check
• GitNexus staged change detection: LOW risk, 0 affected processes

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Summary by cubic

Add a PostHog before_send filter to drop Instagram iOS webview bridge $exception events, reducing noise in error tracking. Real app errors and all other analytics remain intact.

• Bug Fixes
  • Filters only $exception events containing window.webkit.messageHandlers.
  • Hooks before_send in the PostHog client via posthog-js.
  • Adds unit tests and documents the client-side error filtering boundary.

^{Written for commit 52c4478. Summary will update on new commits.}

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