docs(server-changes): document mollifier API mutations on buffered runs

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: d-cs

.server-changes/mollifier-mutations.md

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+---
+area: webapp
+type: feature
+---
+
+Mollifier API mutations on buffered runs: apply tag, metadata, replay, reschedule, cancel, and idempotency-key-reset mutations against buffered runs, with a fallback that mutates the buffer snapshot when the run has not been drained yet.