Introduce structured tracing / metrics surface for observability

[P3az3]

Problem Statement campaign uses raw env.events().publish for client-visible state. There is no internal framework for log lines, error breadcrumbs, or counters. On a faulty production deployment, debugging requires painstakingly reading on-chain events.

Why it matters Soroban hostside tracing is unavailable in production (no eprintln!); internal observability is best expressed through additional public events and typed structs.

Expected Outcome Add a structured event diagnostics_emit (off-by-default, gated behind a feature flag) that captures:

• Latency budget used per call.
• CPU/memory envelope per call.
• Error code on panic.

Also, a public metrics_view(env) -> CampaignMetrics entrypoint that returns diagnostic counters (counters for donate_received_total, donate_panicked_total_by_code, etc.) if a feature flag is enabled.

Acceptance Criteria

• New feature flag diag (default off).
• New event variant diagnostics_emit (gated by the flag).
• New view entrypoint metrics_view.
• Test exercises flag-off (no events emitted) and flag-on (events emitted).
• Documentation in docs/observability.md includes a user-facing JSON schema.

Implementation Notes

• The flag toggle must be cheap; use #[cfg(feature = "diag")].

Affected Files / Modules

• campaign/src/lib.rs
• campaign/Cargo.toml (feature)
• New: docs/observability.md

Dependencies — None.

[Spaully]

I can resolve this issue.

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #31)
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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@Spaully's PR #60 was approved and merged by @P3az3.

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👏 Great work, @Spaully! Keep contributing to MillestoneX.