test(leaks): add heap-snapshot regression harness

[ranvirjrj-beep]

Summary

Adds a deterministic V8 heap-snapshot regression harness for the SDK leak test suite.

• captures heap snapshots before and after repeated scan workloads
• compares retained objects at constructor level
• supports configurable regression thresholds
• requires deterministic GC via --expose-gc
• includes an intentional leak-injection mode to verify that the harness fails on a real regression
• writes a machine-readable heap-diff.json report
• runs the harness in the nightly CI job and uploads the report as an artifact
• documents local usage, thresholds, and failure interpretation

Validation

The harness is designed to:

• pass when retained constructor growth stays within the configured threshold
• fail when deliberate retained-object growth is injected
• produce a reproducible constructor-level diff for CI inspection

Issue

Closes #182

[drips-wave]

@ranvirjrj-beep 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

[truthixify]

Merged. Constructor-level diffing on top of the existing snapshot dump is exactly the delta that was missing, and gating it to nightly keeps PR time sane. Nice work @ranvirjrj-beep.