fix: promote untouched L2 candidates that meet thresholds

[RerankerGuo]

Summary

Fixes #1932.

• Re-check untouched candidate policies after the normal L2 touched-policy gain pass.
• Promote candidates whose stored support/gain already satisfy thresholds, even when the current episode did not associate any trace with them.
• Emit the usual l2.policy.updated event and include the promoted candidate in touchedPolicyIds.

Validation

• npm run lint

Attempted but blocked by the local environment:

• npm test -- --run tests/unit/memory/l2/l2.integration.test.ts -t "untouched candidate"

The targeted test fails before assertions because this environment lacks a usable better-sqlite3 native binding. Rebuilding the binding is blocked here by Node 18 and a compiler that rejects -std=c++20.

[Memtensor-AI]

Automated Test Results: FAILED

Cloud test-engine full-audit rerun against dev-v2.0.22 failed after clearing the cloud workspace disk.

• Run: tr-2482a03a-cc8 on cloud test-engine 10011

• Verdict: fail_code_bug

• Summary: Tests failed. Failed cases:

• No executable phase result recorded.

Do not merge until this is fixed and cloud tests pass.

[Memtensor-AI]

Automated Test Results: ENVIRONMENT RETRY

The previous cloud test-engine run against dev-v2.0.22 did not produce executable test phases because environment preparation failed during npm install (npm install ETIMEDOUT during env prep). This is being treated as a cloud environment/network failure, not a confirmed code failure.

• Previous run was superseded by retry: tr-d505739a-e76 on cloud test-engine 10011
• Branch tested: refs/pull/1994/head

I will post the final pass/fail result when this retry completes.

[Memtensor-AI]

Automated Test Results: PASSED

Cloud test-engine rerun against dev-v2.0.22 completed successfully.

• Run: tr-d505739a-e76 on cloud test-engine 10011
• memos_local_plugin/unit: 9 passed, 0 failed, 0 skipped

Manual code review is still required before merge.