fix: dead code and staticcheck findings

Found by the audit of 2026-09-14 with staticcheck -checks=all.

Removed unused functions (sortedDirs, artistDirOfPlan, alArtistCredit, editorFixesAll, Plan.nameSession, a dead test helper), the ineffective assignment to the never-read Module.contentRows field, a duplicated condition in scan.collectAudio, and two loops staticcheck flagged in internal/audio. staticcheck is now clean.