docs: wanted, dbtest, recommendation and playlists comments say what the code does (#6277)

The same rewrite as d191a282, for four packages whose comments carried the
most history: tuning stories, quotes from the operator, single-install
measurements in library_scale.go, and 'used to' accounts of bugs. The
constraint each story protected stays as a present-tense reason (why tier 3
is reduced not removed, why the randomised arms take an order seed, why the
seed artist is eligible in Songs-like). Comments naming system_cron.go and
config.RecommendationConfig, neither of which holds what they claimed, now
name the scheduler and internal/recsettings.

Checked: non-comment tokens identical to HEAD; gofmt clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/playlists/system_mixes.go

@@ -11,10 +11,9 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 )
 
-// Discovery mixes (#419-423). One generic producer + per-mix spec.
-// Replaces the five near-identical produceXxx functions that all
-// fetched ranked (id, album_id, artist_id) rows, diversified,
-// truncated, and emitted a single playlist.
+// Discovery mixes: one generic producer + a per-mix spec. Each mix fetches
+// ranked (id, album_id, artist_id) rows, diversifies, truncates, and emits a
+// single playlist.
 //
 // Day-keying is a per-mix property captured by `dailyRotate`:
 //
@@ -40,7 +39,7 @@ import (
 // Discover so shuffle-on-play has a varied pool within a day.
 const discoveryMixLen = 100
 
-// Minimum viable mix sizes (issue #1246): below the floor the variant
+// Minimum viable mix sizes: below the floor the variant
 // is withheld entirely so Home renders its "listen more to unlock"
 // placeholder — a playlist with a couple of songs reads as a build
 // bug, not a mix. Album-coherent mixes (NewForYou / FirstListens) get
@@ -68,7 +67,7 @@ type discoveryMixSpec struct {
 
 	// minLen is the minimum viable mix size: a finished pool below it
 	// is withheld (no playlist row) so the client renders the locked
-	// placeholder instead of a mix that looks built-wrong (#1246).
+	// placeholder instead of a mix that looks built-wrong.
 	minLen int
 
 	// fetch returns the raw ranked rows. dateStr is supplied for
@@ -109,10 +108,10 @@ func produceDiscoveryMix(spec discoveryMixSpec) systemPlaylistProducer {
 // so each day's downstream truncate-to-N surfaces a different slice
 // while contiguous-block ordering inside the slice is preserved.
 //
-// Rotation happens WITHIN each contiguous same-pick-kind block
-// (#1267): tiered mixes arrive tier-ordered, and a whole-pool
+// Rotation happens WITHIN each contiguous same-pick-kind block:
+// tiered mixes arrive tier-ordered, and a whole-pool
 // rotation would hoist tier-3 filler above tier-1's exact fits.
-// Untiered pools are a single block, which reduces to the original
+// Untiered pools are a single block, which reduces to a plain
 // whole-pool rotation (same seed, same first draw). Empty /
 // single-element pools pass through unchanged.
 func rotateForDay(pool []discoverTrack, userID pgtype.UUID, dateStr string) []discoverTrack {