fix(roles): stop naming keyboard parts from mixed accompaniment

[cursor]

Closed as proven duplicate of canonical #873

Exact duplicate/supersession evidence was revalidated immediately before closure:

• Canonical feat(roles): wire measured register overlap into section warnings #873 was at 9f1e166da9acf043b7f4b8b6df84b5e1a2417706.
• This PR's exact head 23678f3931dd497dc3c21312544e5843da99a9d4 is a direct fast-forward child of that head: compare(9f1e166d…, 23678f39…) reports ahead_by=1, behind_by=0.
• The single successor commit contains the remaining role-presence/demo-honesty correction: mixed other cannot authorize named keyboard/guitar roles, and demo/browser/shared/Rust fixtures no longer fabricate those identities.
• The canonical feat(roles): wire measured register overlap into section warnings #873 branch cursor/bc-250fad94-8fb4-48fa-b3e0-959ae4494cb3-d696 was fast-forwarded without force to this exact 23678f39… commit and refetched successfully.
• Therefore this PR has no remaining unique semantic or file delta that is not now owned by feat(roles): wire measured register overlap into section warnings #873.

No checks/reviews from this PR are transferred. #873 must regenerate exact-current-head repository and central evidence on 23678f39…; #783/#861/#781/#866 and independent approval remain separate gates.

[cursor]

Review at 23678f39 vs develop

This is the presence + demo-honesty landing for the overlap lineage. Prefer it over draft #873 (9f1e166d). Do not merge #873 in parallel. Do not mix lock-in (#880) or Stem Lab (#884) onto this head.

Verified:

• map_stems_to_roles maps only vocals → lead-vocal and bass → bass-guitar. Mixed other cannot mark Keyboard Left Hand, Keyboard Right Hand, or Acoustic Guitar active.
• test_map_stems_to_roles_other_does_not_name_accompaniment_roles locks that contract.
• Shared demo, browser-fallback, and Rust fixtures replace Density warning / Melodic overlap / named keyboard clash copy with the engine next-action sentence. Keyboard-hand and guitar demo warnings are empty.
• SectionRoadmap asserts the honest bass sentence and rejects the fabricated identity strings.
• Doctoring cites Bregman (1990), Moore (2012), Foote (2000), and ISO 226:2023 and keeps overlap on song -> section -> role windows.

No Critical findings on this head.

Residual, not a merge blocker for this slice: compute_handoffs tests still feed a synthetic keys-left activation. That path can no longer come from map_stems_to_roles. Keep it as a unit of the handoff helper, or drop the synthetic key so the test does not imply a live product path. A dedicated accompaniment role card still needs a later contract; do not rename other into keys or guitar to fill it.

This comment is not approval. Keep the draft until exact-head CI and a qualifying independent non-author last-push review exist.

  

_{Sent by Cursor Automation: Fix Issues}