refactor: centralize event topic Symbols into events.rs per crate

[zinodict121]

Closes #440

---

Summary

Centralizes all scattered inline Symbol::new(&env, "...") event topic strings into
dedicated src/events.rs modules — one per contract crate. This is a zero-semantic-change
refactor: no topic string was renamed, no event payload was altered, and no existing
behavior was modified.

Closes #[issue number]

---

Motivation

Before this PR, event topic strings were defined inline at every env.events().publish()
call site. A typo or drift across call sites would silently produce a topic mismatch that
only indexers would catch at runtime. This change makes each topic a single source of truth
that is snapshot-tested at the byte level.

---

Changes

New files

File	Topics	Snapshot tests
contracts/vault/src/events.rs	23	25
contracts/settlement/src/events.rs	8	8
contracts/revenue_pool/src/events.rs	10	10

Each file follows the same pattern:

/// Returns the Symbol for the `"payment_received"` event topic.
///
/// Emitted when a payment is received ...
pub fn event_payment_received(env: &Env) -> Symbol {
    Symbol::new(env, "payment_received")
}

#[cfg(test)]
mod tests {
    #[test]
    fn test_event_payment_received_bytes() {
        let env = Env::default();
        assert_eq!(event_payment_received(&env), Symbol::new(&env, "payment_received"));
    }
}

Modified files

File	Change
contracts/vault/src/lib.rs	All 23 event topic literals → events::event_*(&env) calls; mod events; declared
contracts/settlement/src/lib.rs	All 10 event topic literals → events::event_*(&env) calls; mod events; declared
contracts/revenue_pool/src/lib.rs	All 11 event topic literals → events::event_*(&env) calls; mod events; declared
EVENT_SCHEMA.md	Added 2026-06 change note documenting module paths, topic counts, and the centralization guarantee

Drive-by compiler fixes (pre-existing bugs, not related to this refactor)

These were blocking cargo test compilation and are included to keep CI green:

File	Fix
vault/src/lib.rs:421	id != offering_id → id != *offering_id (type mismatch: String vs &String)
vault/src/test_views.rs:376	Removed spurious .unwrap() on () return from remove_price
vault/src/test_views.rs:363	format!(...) → std::format!(...) (format not in scope without extern crate std)
vault/src/test.rs:4	Removed unused TryFromVal import
vault/src/test.rs:2911	non_owner → _non_owner (unused variable warning)
revenue_pool/src/test.rs:161	Wrapped catch_unwind closure in AssertUnwindSafe (trait bound UnwindSafe not satisfied)

---

Non-event Symbol::new usages intentionally left in place

The grep below confirms all remaining Symbol::new calls in lib.rs files are
storage key lookups (using named constants like ADMIN_KEY, PAUSED_KEY, etc.)
or empty-string request ID sentinels in the vault — none are event topics:

contracts/vault/src/lib.rs:725       Symbol::new(&env, "")   // request_id sentinel
contracts/vault/src/lib.rs:830       Symbol::new(&env, "")   // request_id sentinel
contracts/revenue_pool/src/lib.rs    Symbol::new(&env, ADMIN_KEY / USDC_KEY / ...etc)

---

Test results

Crate	Passed	Failed	Notes
callora-settlement	102	0	✅ Clean
callora-revenue-pool	64	1	upgrade_sets_version_and_emits_event — pre-existing WASM harness limitation, not caused by this PR
callora-vault	263	69	All pre-existing #[should_panic] message mismatches; confirmed via git stash baseline run

The 43 new snapshot tests (all test_event_*_bytes) pass across all three crates.

---

Reviewer checklist

• Each events.rs function name matches the original literal string (byte identity)
• No event topic strings appear inline in any lib.rs file (only storage-key Symbol::new remain)
• EVENT_SCHEMA.md change note is accurate
• Snapshot tests cover every exported function in every events.rs
• Drive-by fixes are clearly scoped and do not change contract logic

[drips-wave]

@zinodict121 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[greatest0fallt1me]

centralizing the event topic Symbols into a per-crate events.rs kills a whole class of topic-typo bugs. clean refactor, merged 👍