Suggestions: turn artists away by the DJ's fading rule (MAD-762)

[madeofpendletonwool]

Closes MAD-762.

The DJ's half of MAD-762 already landed with MAD-756 (#47). It added avoidBelow = -0.25 and TestProfileSkipsFade with the issue's three cases. This PR does the other half: search suggestions (suggest.TurnedAway, ADR 0010) still turned an artist away for any skip, with no decay.

What changed

• suggest.TurnedAway(snap, history, now) now returns dj.NewProfile(...).Avoid. Suggestions and the DJ now share one rule:
  • A skip counts more the sooner it comes.
  • It fades with a 2-hour half-life.
  • It's weighed against full listens, queued songs and hearts.
  • An artist is turned away only below −0.25.
• Artists are compared by dj.ArtistKey (match.Simplify), the way the DJ keys them.
• ADR 0010 and ADR 0012 describe the shared rule. The "unfaded rule until MAD-759" note is gone. MAD-759 still moves the rest of suggestions onto the DJ.

Behavior change

Under the old rule, one full listen brought a skipped artist back. Under the DJ's rule, a quick skip (−1.5) outweighs one full listen (+1) until it fades or the room plays them again. This matches what autopilot already does.

Tests

• The suggest test env ran the service on the wall clock while its history used a fixed clock. That only worked because skips never faded. The service now uses the test clock.
• New TestSkipFades: two days after a skip, the artist is suggested again.
• TestSkipMemory now checks that two full listens outweigh a quick skip.
• go test -race ./... and golangci-lint (0 issues) pass. There are no web changes.

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