PR #11889 CodeRabbit re-review: change per-file failure log warn->debug

CodeRabbit posted new Major finding at 2026-06-08T14:49:09Z on PR #11889 (BOLT-1213). The per-file download failure log at downloadFailures.ts:160 currently logs at warn level; CodeRabbit says this is too noisy for first-time transient misses and blurs the actual alert signal (which is the warn at line 186 for evaluated repeated/stuck state). Recommendation: change line 160 warn->debug, keep warn at 186. Single-line fix.