perf(test): add scanner leak detection harness

[israelolrunfemi]

Close: #26

• add RSS slope-based leak regression test
• capture heap snapshots during scan loop
• add profiling scripts and nightly CI workflow
• document heap snapshot debugging workflow

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[truthixify]

The leak harness itself is exactly what issue #26 asked for: v8 heap snapshots over iterations, RSS slope detection, the leak-scan.yml CI workflow, and the docs/scan-leak-debugging.md companion. Keep this work intact.

Blocker is the base. This PR was opened against an older main and the current diff would undo:

• CONTRIBUTING.md from docs: add SDK contributing policy #13
• JSDoc additions across all stellar/* files from docs: expand Stellar public API JSDoc #14
• The time-range / cursor / RetentionExceededError changes from feat: add Stellar announcement range scans #16
• The test/chains/stellar/announcements.test.ts test file from feat: add Stellar announcement range scans #16

Could you rebase onto develop?

git fetch origin
git rebase origin/develop
# resolve in:
#   CONTRIBUTING.md (keep develop's version)
#   src/chains/stellar/* (keep develop's JSDoc + range-scan APIs)
#   test/chains/stellar/announcements.test.ts (keep develop's version)
# add your leak-harness files (test/leak/*, docs/scan-leak-debugging.md, .github/workflows/leak-scan.yml)
git push --force-with-lease

Thanks @israelolrunfemi, the harness is real and we want it in.