test: add runtime and memory stress coverage

[zefutoff]

Summary

• add sustained real-engine runtime stress coverage
• exercise hundreds of repeated grammar analyses and spelling checks
• measure retained QuickJS memory after explicit garbage collection
• cover concurrent bursts sharing the serialized QuickJS runtime
• cover repeated engine creation, use and shutdown cycles
• expose an internal QuickJS memory snapshot for engine tests
• add an opt-in grammalecteStress Gradle property
• add make stress-check
• run runtime stress checks in the existing CI quality job
• document the initial QuickJS memory baseline and regression budget
• complete the final Phase 4 roadmap item

Runtime stress coverage

The suite verifies:

• 400 sustained long-lived analyses
• repeated spelling checks during sustained use
• concurrent grammar/spelling calls from 8 workers
• repeated QuickJS engine lifecycle cycles
• retained QuickJS memory growth after garbage collection

Memory baseline

Initial development measurement:

• memory before stress: 30.00 MiB
• memory after stress: 30.00 MiB
• retained-memory growth: 0.00 MiB
• allocated QuickJS memory after stress: 40.62 MiB
• objects: 91,502
• strings: 36,620

The retained-memory growth budget is 8 MiB.

The measurement covers QuickJS runtime memory in the JVM test environment and is not a total Android process-memory claim.

Validation

• make check
• make performance-check
• make stress-check
• make assemble
• git diff --check
• final APK does not request android.permission.INTERNET

Roadmap

Completes:

• Add memory/runtime stress tests

This completes Phase 4 — Product polish.