DiagnosticEventWalker: pinpoint failing contracts in call chains (#339)

[Marvelg256]

Summary

Implements DiagnosticEventWalker — a chronological event walker that maintains a contract call stack to identify the exact contract that caused a failure in multi-contract Soroban transactions.

Closes #339

Problem

When a multi-contract Soroban transaction fails (e.g., Contract A → B → C, where C panics), the raw result only shows the top-level invocation failed. Grat would mistakenly blame Contract A for errors deep in Contract C, sending developers on wild goose chases.

Solution

A new module crates/core/src/decode/event_walker.rs provides DiagnosticEventWalker with a locate_failing_contract() method that:

1. Walks events chronologically — processes DiagnosticEvent objects in the order the VM emitted them
2. Maintains a call stack — pushes callee contract IDs on fn_call system events, pops on fn_return
3. Detects failures via multiple indicators:
   • in_successful_contract_call = false (VM flag)
   • ScVal::Error payload (HostError/ContractError return values)
   • Topic keywords: error, panic, revert, hosterror, failed, trap
4. Halts on first failure — returns the contract ID at the top of the stack at the exact moment of failure
5. Handles edge cases — first-instruction panics (empty stack fallback to the event's own contract_id), fn_call without contract_id, nested calls with returns

Files Changed

• crates/core/src/decode/event_walker.rs (new) — 413 lines implementing the walker with 16 unit tests
• crates/core/src/decode/mod.rs — added pub mod event_walker;

Testing

All 252 tests pass, including 16 new tests covering:

• Direct failure detection (A calls B, B panics → returns B)
• Nested chain failure (A → B → C, C panics → returns C)
• First-instruction panic (empty stack fallback)
• No-failure scenarios (returns None)
• Chronological halt (first failure wins)
• All failure indicators (error, panic, revert, hosterror, ScVal::Error, failed, trap)
• Stack discipline (push on fn_call, pop on fn_return)
• System events without contract_id (no stack corruption)