feat(ci): add deep invariant tests with notifications

[howydev]

Summary

Adds a new CI workflow for deep invariant testing against Rust precompiles,
running twice weekly with Slack notifications on failure.

Changes

• New workflow: .github/workflows/invariant-deep.yml
  • Runs all invariant tests: FeeAMMInvariantTest, StablecoinDEXInvariantTest,
    TempoTransactionInvariantTest
  • Tests against Rust precompiles via tempo-foundry (not Solidity
    implementations)
  • Schedule: Thursday & Sunday at 3 AM UTC (11 AM Singapore)
  • Target runtime: ~60 minutes (1500 runs × 1500 depth)
  • Hard timeout: 120 minutes to prevent overbilling
  • Manual trigger available via workflow_dispatch

Features

• ✅ Slack notifications on scheduled failures (requires SLACK_WEBHOOK_URL
  secret)
• ✅ Failure log uploads for debugging
• ✅ Success tracking job for dashboard visibility
• ✅ Rust backtrace enabled for better error diagnostics
• ✅ Uses Depot 8-core runners for fast builds

Cost

• Estimated: $187/year ($16/month)
• Calculation: 104 runs/year × 60 min × $0.03/min
• Worst case (120 min timeout): ~$374/year