feat(error-tracking): standardize exception event metadata

[hpouillot]

💡 Motivation and Context

Standardize iOS exception events with the cross-SDK metadata contract so manual errors and deferred native crashes emit consistent severity, source, mechanism, provenance, and linkage metadata.

Canonical contract: https://github.com/PostHog/sdk-specs/blob/main/openspec/specs/exception-event-metadata/spec.md

💚 How did you test it?

• Full iOS test suite: 763 tests passed
• Updated event-shape fixtures

make format is blocked locally because the installed SwiftFormat version does not support the repository's wrapIfStatementBodies rule.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Implemented with pi from the shared OpenSpec contract. Existing event shapes remain compatible while canonical metadata is added at the narrowest capture boundary.

[github-actions]

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