feat: detect excessive Soroban event topics

[Joeloo1]

Summary

• Adds detectExcessiveEventTopics() detector in rules/stellar/events/
• Flags env.events().publish() calls whose topic tuple exceeds 4 entries or embeds large payload types (Bytes, BytesN, String, Vec, Map) directly in topics
• Uses brace-counting to correctly parse nested function calls inside topic tuples

Test plan

• Events with more than 4 topics are flagged
• Events with Bytes, String, Vec, or Map in the topic tuple are flagged
• Events with ≤ 4 plain topics are not flagged
• Contracts with no event calls return detected: false
• Fixture stellar-excessive-event-topics.json matches detector output

Closes #372

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