Add structured logging event StructuredLogEntry to events.rs with stable schema id

[greatest0fallt1me]

Description

events.rs emits many domain events but observability tooling has no stable, machine-parseable schema. Introduce a StructuredLogEntry event with a schema_id: Symbol, a severity enum (Info/Warn/Error), and a typed fields map so off-chain indexers can decode entries deterministically. Wire it as the low-level primitive that higher-level emitters delegate to.

Requirements and Context

• New StructuredLogEntry event type in events.rs with a schema_id registry constant
• Emitter helper EventEmitter::emit_structured(env, schema_id, severity, fields)
• Backwards-compatible — existing typed events continue to fire
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b feature/structured-log-entry-events

2. Implement changes
   • contracts/predictify-hybrid/src/events.rs
   • contracts/predictify-hybrid/src/tests/structured_log_entry_tests.rs
   • Document schema ids in docs/
3. Test and commit
   • cargo test -p predictify-hybrid structured_log_entry
   • Cover edge cases
   • Include test output and notes in the PR

Example commit message

feat: add StructuredLogEntry event primitive with stable schema id

Acceptance Criteria

• StructuredLogEntry is emittable from at least three existing call sites
• schema_id constants are exported and documented
• Decoder roundtrip test passes for every registered schema id

Guidelines

• NatSpec-style /// doc comments on all new public items
• Clear documentation and inline comments
• Timeframe: 96 hours

[Adesam007-pr3dator]

@Adesam007-pr3dator has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

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1. Open a Pull Request referencing this issue (e.g., Closes #651)
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