Stabilize modulation chart rerender checks

[itsDNNS]

Summary

• replace fixed sleeps in modulation direction and range rerender checks with a deterministic rendered-chart condition
• distinguish downstream results by render provenance and 30-day results by chart sample count
• retain the chart layout assertion and add a normal-CI contract against reintroducing fixed waits

Closes #740

Tests

• focused contract and modulation static tests
• focused Chromium rerender checks
• repeated focused Chromium checks (10 iterations, 20 assertions)
• complete non-E2E test suite
• complete browser E2E suite
• Python compilation and git diff --check