feat(core): trace diffing — diffTraces() API

[sepehr-safari]

Summary

Implements diffTraces() — a new core API for comparing two parsed traces and surfacing differences at multiple levels:

API

export function diffTraces(a: ParseResult, b: ParseResult): TraceDiff;

TraceDiff Output

• onlyInA / onlyInB — Events present in only one trace (by messageId)
• modified — Field-level differences for events present in both:
  • timestamp, direction, action, payload (deep equality), errorCode
• failuresOnlyInA / failuresOnlyInB — Failures detected in one but not the other
• summaryDiff — Session summary differences (event count, failure count, duration, status, transaction ID)

Implementation Details

• Events matched by messageId (OCPP UniqueId)
• Handles Call + CallResult pairs sharing the same messageId (position-based comparison)
• Deep equality for payload comparison (JSON-serializable data only)
• Pure function — no I/O, no side effects, browser-safe
• Exported from @ocpp-debugkit/toolkit/core and root barrel

Files Changed

• packages/toolkit/src/core/diff.ts — new module (implementation + types)
• packages/toolkit/src/core/diff.test.ts — 13 unit tests
• packages/toolkit/src/core/index.ts — export diffTraces + types
• packages/toolkit/src/index.ts — re-export from root barrel
• .changeset/v03-trace-diffing.md — minor bump changeset
• CURRENT_STATE.md — updated with v0.3.0 progress

Closes #85

Checklist

• diffTraces() implemented with full TraceDiff return type
• Events matched by messageId, with field-level diff for modified events
• Failures diffed by code
• Summary diffed with human-readable difference strings
• Unit tests: identical, completely different, partially different, missing events, modified payloads, timestamp differences
• Pure function — no I/O, no side effects, browser-safe
• Exported from @ocpp-debugkit/toolkit/core and root barrel
• Changeset added
• CURRENT_STATE.md updated
• Lint, typecheck, test, format all pass (322 tests)